Development and psychometric evaluation of a newpatient-reported outcome instrument measuringthe functional impact of insomnia

Christopher Bell • Lori D. McLeod •

Lauren M. Nelson • Sheri E. Fehnel •

Laurie J. Zografos • Brian Bowers

Accepted: 5 March 2011 / Published online: 20 April 2011

� Springer Science+Business Media B.V. 2011

Abstract

Purpose The objective of this study was to develop and

validate a patient-reported outcome instrument to com-

prehensively assess the consequences of inadequate sleep

for use in insomnia-related studies.

Methods To inform item development, relevant constructs

were identified through patient focus groups, literature review,

and expert input. Following a translatability assessment for

United States (US) English, US Spanish, and French, the draft

items were refined through iterative sets of patient interviews

in the United States and France. Psychometric properties were

evaluated using patient responses from a validation study

including 432 participants with either a diagnosis of primary

insomnia or no history of insomnia.

Results Psychometric analyses supported item reduction

from 38 to 26 items, yielding a unidimensional scale and

preserving the original content (mood, tiredness/energy,

memory/concentration, motivation, daily performance,

social interaction, sexual functioning). Evidence of internal

consistency (coefficient a = 0.97), convergent validity,

and known-groups validity also was documented.

Conclusions The Sleep Functional Impact Scale (SFIS) is

a psychometrically sound measure targeting the impact of

insomnia on patient functioning. When administered with a

sleep diary, this instrument has the ability to provide a

more comprehensive assessment of treatment response in

clinical studies.

Keywords Insomnia � Questionnaire � Impact �Psychometrics � Patient-reported outcome � PRO

Abbreviations

ANOVA Analysis of variance

DSM-IV-TR Diagnostic and Statistical Manual of

Mental Disorders, Fourth Edition,

Text Revision

EFA Exploratory factor analysis

ESS Epworth Sleepiness Scale

FOSQ Functional Outcomes of Sleep

Questionnaire

IRT Item response theory

ISI Insomnia Severity Index

KN Knowledge Networks

MOS-Sleep Medical Outcomes Study Sleep Scale

PRO Patient-reported outcome

PSQI Pittsburgh Sleep Quality Index

SFIS Sleep Functional Impact Scale

US United States

WPAI–GH Work Productivity and Activity

Impairment–General Health Questionnaire

Introduction

Insomnia, the most common sleep disturbance reported in

the United States (US), is characterized by difficulty falling

Disclaimer: Funding for this study was provided by

GlaxoSmithKline, Research Triangle Park, NC. All listed authors

meet the criteria for authorship set forth by the International

Committee for Medical Journal Editors.

Electronic supplementary material The online version of thisarticle (doi:10.1007/s11136-011-9885-8) contains supplementarymaterial, which is available to authorized users.

C. Bell (&) � B. Bowers

GlaxoSmithKline, Research Triangle Park, NC, USA

e-mail: Christopher.f.bell@gsk.com

L. D. McLeod � L. M. Nelson � S. E. Fehnel � L. J. Zografos

RTI Health Solutions, Research Triangle Park, NC, USA

123

Qual Life Res (2011) 20:1457–1468

DOI 10.1007/s11136-011-9885-8

asleep, difficulty maintaining sleep, waking too early in

the morning, and/or an inability to experience restorative

sleep [1–3]. Prevalence estimates in the United States vary

from 2 to 42.5%, primarily because researchers have

operationally defined the insomnia population in different

ways across studies [2, 4, 5]. When insomnia cannot be

attributed to a medical or psychiatric condition or to dis-

rupting environmental factors, it is referred to as primary

insomnia [6], with the prevalence of this more narrow

definition ranging between 6 and 10% of the US adult

population [3].

In addition to the nighttime factors related to sleep

disturbance, insomnia leads to a severe decline in func-

tioning during the day. These daytime aspects related to

functional and emotional impairment coupled with the

nighttime characteristics are reflected in the Diagnostic

and Statistical Manual of Mental Disorders, Fourth Edi-

tion, Text Revision (DSM-IV-TR) criteria for diagnosing

insomnia. Given the inclusion of daytime functioning in

the DSM-IV-TR criteria, it is important to measure this

aspect for diagnostic purposes. Furthermore, significant

changes in daytime functioning can be a convincing out-

come when comparing medical interventions for the

management of insomnia. However, there are no patient-

reported outcome (PRO) measures designed to evaluate

daytime functioning that meet current regulatory require-

ments as described in the Food and Drug Administration’s

guidance, Patient-Reported Outcome Measures: Use in

Medical Product Development to Support Labeling Claims

[7]. In addition, the European Medicines Agency’s draft

guidance on insomnia recommends evaluating daytime

functioning in clinical trials of new drugs to treat

insomnia [8]. Given these demands, there is a need for an

instrument that meets regulatory requirements and can

evaluate the aspects of daytime functioning associated

with insomnia.

This article focuses on a promising new PRO instru-

ment, the Sleep Functional Impact Scale (SFIS), which was

developed for use in clinical trials of new treatments for

insomnia. The SFIS specifically targets the impact of pri-

mary insomnia on patients’ subjective experiences with

their psychological and cognitive functioning, social

activities, and work productivity utilizing a weekly refer-

ence period.

This article presents the development and psychometric

evaluation of the SFIS, including the qualitative research

completed to identify concepts relevant to the functional

impact of insomnia, cognitive debriefing interviews to

refine the item wording, and the quantitative evaluation of

the psychometric properties based on data from a group of

adults who self-reported symptoms of primary insomnia

and another group of adults who reported no such

problems.

Methods

The SFIS was developed and evaluated in three sequential

stages: (1) qualitative research using patient focus groups, a

literature review, and expert panel input to generate the

initial set of items for a draft questionnaire; (2) cognitive

debriefing to optimize question wording and response

scales; and (3) psychometric evaluation. All stages were

conducted in the United States (US) with the exception of

cognitive debriefing, where interviews conducted in the

United States were augmented with interviews conducted

in Lyon, France. Relevant ethics and institutional review

board approvals were obtained, and all participants pro-

vided written informed consent.

Item generation and draft questionnaire

Key SFIS constructs were elicited during five focus groups

conducted in the United States. Patients were recruited

directly from physicians’ offices. Focus group eligibility

was determined on the basis of the following inclusion

criteria: physician-confirmed, primary insomnia diagnosis

of 3–6 months’ duration before screening; age 18–64 years;

fluent in English; and capable of participating in a 2-hour

(2-h) focus group. Patients were not eligible for participa-

tion if their sleep disturbances were due to the comorbidities

known to interfere significantly with sleep (e.g., depression,

generalized anxiety disorder, circadian rhythm disorder,

narcolepsy, restless leg syndrome, and chronic pain syn-

dromes); the direct psychological effects of alcoholism or

drug addiction; or the direct effects of a medication. The use

of sleep medications was not part of the inclusion or

exclusion criteria. However, the use of such medications was

discussed in the focus groups, with respondents reporting use

of common over-the-counter (e.g., acetaminophen and

diphenhydramine HCl [Tylenol PM]) and prescription

(e.g., zolpidem and eszopiclone) sleep medications. Table 1

provides demographic information for the 49 individuals

participating in the focus groups.

Two experienced moderators conducted each focus

group using a semi-structured discussion guide, with the

goal of having patients describe, in their own words, their

experiences with insomnia (e.g., general experiences,

symptoms, and direct and functional impacts). Each session

was audio-recorded to facilitate transcription and sub-

sequent analysis. From the results of the focus groups, a

targeted search of the relevant literature, input from a six-

member advisory panel of sleep specialists (see Acknowl-

edgements), and an initial translatability assessment (US

English, US Spanish, and French), a 50-item draft version

of the SFIS was created. The draft SFIS reflected a set of

questions that were meaningful and relevant to participants

when evaluating the impact of insomnia.

1458 Qual Life Res (2011) 20:1457–1468

123

Cognitive debriefing interviews

The draft SFIS was tested via face-to-face cognitive

debriefing interviews of patients with primary insomnia in

the United States and France. The purpose of the interviews

was to examine the practicality, as well as to support the face

and content validity of the SFIS. A total of 46 patients par-

ticipated in two rounds of face-to-face interviews (Table 1),

representing the three target languages: US English (13

interviews), US Spanish (16 interviews), and French (17

interviews). Patients were recruited using the same meth-

odology implemented for the focus groups and were required

to meet the same inclusion criteria, with the exception that

patients had to be fluent in the language for which they were

recruited. Patients were asked to complete the draft SFIS in

the presence of an interviewer and to clarify the compre-

hension and response process. Item reduction based on the

results of this qualitative research yielded a 38-item ques-

tionnaire reflecting the impact of insomnia on the following

aspects: mood (11 items), tiredness/energy (4 items),

memory/concentration (4 items), motivation (6 items), daily

performance (8 items), social interaction (4 items), and

sexual functioning (1 item). A 5-point Likert-type scale was

used for all questions relating to quantity (‘‘Much worse than

usual’’ to ‘‘Much better than usual’’), impact (‘‘Not at all’’ to

‘‘Extremely’’), and frequency (‘‘None of the time’’ to ‘‘All of

the time’’). The recall period throughout the questionnaire

was the past 7 days.

Quantitative psychometric evaluation

The psychometric properties and item performance of the

SFIS were evaluated in a large, Web-based observational

study of US patients. From a Knowledge Networks (KN)

online panel, 32,000 men and women aged 18 years and

older residing in the United States were randomly selected

for potential participation in this cross-sectional, popula-

tion-based survey. The KN panel, a proprietary Web-

enabled panel of research subjects who have agreed to

participate in ongoing survey research, is based on a ran-

dom-digit-dialing sample of the full US population (i.e., a

probability-based panel), ensuring that the panel is repre-

sentative of the US population. KN provides panelists

‘‘points’’ that can be redeemed for cash or Web-enabled

technology at regular intervals for panel participation.

KN administers an annual survey to gather and update

health and demographic information. Individuals who were

pre-identified from the panelists’ health profiles as having

insomnia were selected as candidates for the insomnia

group and those who were not pre-identified were candi-

dates for the control group. Screening questions were tar-

geted for each group to ensure participants met inclusion

Table 1 Qualitative development: study sample characteristics

Focus group Cognitive debriefing interviews–round 1 Cognitive debriefing interviews–round 2

US English US Spanish French US English US Spanish French

Sample size (n) 49 8 11 12 5 5 5

Female (%) 73.5% 87.5% 72.7% 58. 3% 100.0% 60.0% 80.0%

Age, mean (min, max) 46 (18–68) 49 (18–61) 46 (29–60) 43 (32–55) 47 (43–52) 30 (23–36) 44 (32–55)

Race (%)

White 63.30% 87.50% 0.00% N/A 40% 0.00% N/A

Other 37.70% 12.50% 100.0% N/A 60% 100.0% N/A

General health

Excellent 18.40% 12.50% 27.30% 41.70% 0.00% 0.00% –

Very good 30.60% 0% 36.40% 0.00% 60.00% 20.00% –

Good 36.70% 75.00% 27.30% 50.00% 40.00% 40.00% –

Fair 14.30% 12.50% 0.00% 8.30% 0.00% 20.00% –

Poor 0.00% 0.00% 0.00% 0.00% 0.00% 20.00% –

Insomnia severity

Very mild 10.20% 0% 9.10% 0.00% 0.00% 20.00% –

Mild 10.20% 12.50% 0.00% 8.30% 0.00% 0.00% –

Moderate 53.10% 37.50% 27.30% 66.70% 100% 40.00% –

Severe 18.40% 25% 54.50% 25.00% 0.00% 40.00% –

Very severe 2.00% 25% 0.00% 0.00% 0.00% 0.00% –

Unknown 6.10% 0% 9.10% 0.00% 0.00% 0.00% –

– Data are unavailable

n Sample size, US United States

Qual Life Res (2011) 20:1457–1468 1459

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criteria for the study. Panelists eligible for the insomnia

group met the criteria described in Table 2.

In addition to the criteria described in Table 2, eligible

participants from both groups indicated that they had not

been diagnosed with a specific sleep disorder (e.g., circa-

dian rhythm sleep disorder, narcolepsy, parasomnia, peri-

odic limb movement disorder, restless legs syndrome, or

sleep apnea), or specific medical conditions with associated

sleep impairment, such as bipolar disorder, cancer, chronic

fatigue syndrome, congestive heart failure, dementia or

Alzheimer’s disease, HIV/AIDS, lupus, Parkinson’s dis-

ease, rheumatoid arthritis, or schizophrenia. Further study

inclusion criteria required eligible participants to consume

fewer than six caffeinated drinks per day and, if employed,

to work the day shift. Additionally, recruitment rules

specified that 75% of the enrolled participants were

employed at least 20 h per week.

Measures

The Web-based survey included the 38-item draft SFIS

(Electronic supplementary material) along with several

additional sleep questionnaires and a questionnaire designed

to measure health-related impacts on work productivity and

activities. Table 3 provides summary information for these

measures and each is described briefly below.

Medical Outcomes Study Sleep Scale (MOS-Sleep)

The MOS-Sleep is a 12-item, self-report instrument that

assesses both sleep duration and sleep quality. The MOS-

Sleep yields an index characterizing the severity of sleep

problems, as well as six sleep attribute scale scores

addressing Sleep Initiation, Maintenance, Perceived Ade-

quacy, Regularity, Daytime Somnolence, and Respiratory

Impairment [9]. Respondents are asked to recall their sleep

patterns over the past 4 weeks. Hays and Stewart [9] pro-

vide a detailed scoring algorithm that entails rescaling the

original numeric values to a 0 to 100 scale. This step

transforms the ranges for the summary index and six

subscales to 0–100, such that each score represents a per-

centage of the total possible points. Higher scores reflect

more of the attribute. Hays and colleagues [10] extensively

validated the MOS-Sleep in disease-specific clinical trials

and in several nonclinical samples and found acceptable

psychometric properties including reliability, validity, and

responsiveness.

Epworth Sleepiness Scale (ESS)

The ESS is an eight-item, self-report instrument designed

to measure sleep propensity in two domains: level of

daytime sleepiness and somnolence [11]. Respondents are

instructed to refer to their ‘‘usual way of life in recent

times’’ while answering the items. A composite is com-

puted as the sum of nonmissing items and total scores

range from 0 to 24. Higher scores indicate greater levels of

daytime sleepiness. Although the ESS is widely used in

sleep apnea and sleep disorder populations, Milten and

Hanly [12] report reservation regarding the reliability and

responsiveness of the ESS.

Functional Outcomes of Sleep Questionnaire (FOSQ)

The FOSQ is a 30-item, self-report instrument designed to

evaluate the impact of disorders of excessive sleepiness on

five factors related to activities of daily living: Activity

Level, Vigilance, Intimacy and Sexual Relationships, Gen-

eral Productivity, and Social Outcome [13]. Respondents are

asked to rate the difficulty they have generally performing a

given activity. Five subscales (ranging from 0 to 20) and a

global measure (ranging from 0 to 100) are reported, with

higher scores associated with greater function.

Weaver and colleagues [13] reported psychometric

properties of the FOSQ via a study with individuals with no

sleep problems, patients seeking medical attention for a

sleep problem, and patients with diagnosed obstructive

sleep apnea. A factor analysis confirmed the five subscales;

internal reliability was found to be excellent for both

subscales and the total measure, and test–retest reliability

Table 2 Psychometric evaluation: enrollment criteria

Insomnia Control

Reported an insomnia diagnosis from a physician Reported no diagnosis (current or former) of insomnia

Reported trouble falling asleep at the time of screening that

started over 3 months prior to interview for at least

3–5 nights per week

Reported not currently experiencing trouble falling asleep

more than 1 day per week and not currently using a sleep

aid more than 1 time per week

Reported feeling tired during the day at least 3–5 days per week

Reported sleep problems that were at least moderately bothersome

Reported insomnia that was not explained by a physical condition

(e.g., reflux, pregnancy, pain, breathing problems), or an emotional condition

1460 Qual Life Res (2011) 20:1457–1468

123

yielded coefficients ranging from r = 0.81 to 0.90. The

study also found that the FOSQ successfully discriminated

between normal sleep subjects and those seeking medical

attention for a sleep problem.

Pittsburgh Sleep Quality Index (PSQI)

The PSQI is a 19-item, self-rated questionnaire that assesses

sleep quality and disturbances over a 1-month time interval

[14]. Seven composite scores measure Subjective Sleep

Quality, Sleep Latency, Sleep Duration, Habitual Sleep

Efficiency, Sleep Disturbances, Use of Sleeping Medica-

tion, and Daytime Dysfunction. The composite scores are

summed yielding a global measure (range 0–21). Buysse

and colleagues [14] assessed the psychometric properties of

the PSQI over an 18-month period, with a control group of

individuals with normal sleep and a group of individuals

with either depression or sleep disorders, and reported

acceptable measures of internal consistency reliability, test–

retest reliability, and validity.

Insomnia Severity Index (ISI)

The ISI is a brief, seven-item, self-report instrument

designed to measure a patient’s perception of his or her

insomnia based on the severity of sleep-onset and sleep-

maintenance difficulties [15]. Respondents are asked to

reflect over the last 2 weeks. A total score, ranging from 0

to 28, is formed by summing over the nonmissing items.

Higher scores suggest more severe insomnia. Bastien and

colleagues [15] evaluated the psychometric properties of

the ISI and reported adequate internal consistency, indi-

cating the ISI is a reliable self-report measure to evaluate

perceived sleep difficulties, and concluded that the ISI is

sensitive to changes in perceived sleep difficulties with

treatment. Additionally, they reported high agreement

between patient- and clinician-rated scores.

Work Productivity and Activity Impairment–General

Health Questionnaire (WPAI-GH)

The WPAI–GH is a six-item instrument that measures the

percentage of work time missed due to health (absentee-

ism), productivity impairment while working (presentee-

ism), activity impairment, and loss of work productivity due

to health problems. The WPAI–GH uses a recall period of

the past 7 days. Scores are reported as percentages (0–100),

with higher percentages indicating greater impairment and

less productivity [16, 17].

Analysis

Descriptive statistics

Response distributions for the SFIS were examined for the

survey overall and separately for the two classifications

(i.e., insomnia and controls) to identify any potential floor

or ceiling effects and other response anomalies. Item-level

descriptive statistics (e.g., mean, standard deviation) were

computed for the SFIS items overall and separately for the

insomnia and control groups. Item-level effect size esti-

mates (e.g., Cohen’s d) were calculated between the

insomnia and control group.

Correlational analyses

Inter-item and item-total correlational analyses were con-

ducted to examine relationships among items and identify

Table 3 PRO assessments administered

Instrument

(# of Items)

Reference

period

Domains

MOS-Sleep (12) Past 4 weeks Sleep problems index

Sleep disturbance

Sleep adequacy

Daytime somnolence

Snoring

Respiratory impairment

Sleep quantity

ESS (8) Recent times Daytime sleepiness

Somnolence

FOSQ (30) Generally Impact on:

Activity level

Vigilance

Intimacy and sexual relationships

General productivity

Social outcome

PSQI (19) 1 month Subjective sleep quality

Sleep latency

Sleep duration

Habitual sleep efficiency

Sleep disturbances

Use of sleeping medication

Daytime dysfunction

ISI (7) Last 2 weeks Total severity

WPAI–GH (6) Past 7 days Absenteeism

Presenteeism

Work productivity loss

Activity impairment

ESS Epworth Sleepiness Scale, FOSQ Functional Outcomes of Sleep

Questionnaire, ISI Insomnia Severity Index, MOS-Sleep Medical

Outcomes Study Sleep Scale 12, PRO Patient-reported outcome,

PSQI Pittsburgh Sleep Quality Index, WPAI–GH Work Productivity

and Activity Impairment–General Health Questionnaire

Qual Life Res (2011) 20:1457–1468 1461

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potential candidates for revision or removal from the SFIS.

First, inter-item polychoric correlations among the 38 SFIS

items were examined to identify important relationships.

Item pairs that produced an extremely high correlation

(e.g., greater than r = 0.80) were reviewed for redundancy.

Second, item-total correlations were computed for the 38

SFIS items for the entire sample and for the insomnia and the

control group.

Exploratory factor analysis

An exploratory factor analysis (EFA) was employed to

investigate the structure of the SFIS. Specifically, EFA was

performed using the inter-item polychoric correlation matrix

with data from the entire sample, and, as a sensitivity anal-

ysis, the matrix was computed by group. The size of the

eigenvalues [18] and the scree test results [19] from the

principal components analysis guided the choice of the

dimensionality of the tested models. Exploratory factor

analyses retaining varying numbers of factors were con-

ducted. Squared multiple correlations were used as commu-

nality estimates, with unweighted least squares estimation.

The initial orthogonal rotation (varimax) was followed with

an oblique rotation (quartimin), allowing correlation of the

factors. The number of factors selected was based on the size

and pattern of the factor loadings and the interpretability of

the factors. As a sensitivity analysis, the EFA was also

performed using data from the insomnia group alone.

Item Response Theory (IRT) Analysis

The structure of the SFIS was further examined through IRT

analysis to compute item parameter estimates describing the

relation between each item and the latent construct under-

lying the SFIS scale (MULTILOG v. 7.0 [20]). Samejima’s

[21] graded response model was fitted to assess the sensi-

tivity and discriminating ability of each item of the SFIS to

inform item reduction, subscale refinement, and possible

weighting techniques for scoring the final questionnaire.

Reliability and Validity

The internal consistency for the proposed subscales of the

SFIS was evaluated by computing Cronbach’s [22] coeffi-

cient alpha using item-level data. Correlations between the

SFIS and the other sleep-related instruments were computed.

It was hypothesized that the SFIS would moderately corre-

late (r [ |0.40|) with other measures of insomnia—helping

to demonstrate convergent validity. With respect to diver-

gent validity, it was expected that the SFIS would correlate

more highly with scales having similar domain content, such

as the FOSQ and ISI, than with instruments addressing the

mechanics of sleep (e.g., MOS-Sleep and PSQI).

Analysis of variance (ANOVA) was used to examine

mean differences in SFIS scores between individuals with

insomnia and those in the control group, providing evi-

dence of known-groups validity. Planned analyses also

included comparisons based on the number of hours of

sleep per night reported by participants on the MOS-Sleep

(i.e., a report of 7 to 8 h of sleep per night were considered

in the normal or adequate range and those outside would be

compared alongside) and groups categorized using the

global score on the PSQI (i.e., a score greater than 5 was

used to distinguish subjects with poor sleep quality).

Results

Sample demographics

Table 4 summarizes the demographic characteristics of the

432 participants by insomnia and control group. Enrolled

participants in the insomnia group ranged in age from 27 to

81 years, with a mean age of 53.1 (Standard deviation

(SD) = 11.4), while participants in the control group ran-

ged from 19 to 87 years of age, with a mean age of

46.7 years (SD = 17.5). The majority of participants in the

insomnia group were women (n = 140 [81.9%]), and

Table 4 Psychometric evaluation: study sample demographics

Demographic information Insomnia

(n = 171)

Control

(n = 261)

Mean age in years (SD) 53.1 (11.4) 46.7(17.5)

Gender

Male (%) 31 (18.1) 143 (54.8)

Female (%) 140 (81.9) 118 (45.2)

Race

White, Non-Hispanic (%) 164 (95.9) 186 (71.3)

Black, Non-Hispanic (%) 0 29 (11.1)

Other, Non-Hispanic (%) 3 (1.75) 10 (3.8)

Hispanic (%) 1 (0.6) 31 (11.9)

Multiracial, Non-Hispanic (%) 3 (1.75) 5 (1.9)

Marital status

Married (%) 115 (67.2) 159 (60.9)

Single (%) 20 (11.7) 76 (29.1)

Divorced (%) 25 (14.6) 18 (6.9)

Widowed (%) 9 (5.3) 3 (1.2)

Separated (%) 2 (1.2) 5 (1.9)

Education

Less than high school (%) 3 (1.8) 23 (8.8)

High school (%) 30 (17.5) 85 (32.6)

Some college (%) 62 (36.3) 78 (29.9)

College degree (%) 76 (44.4) 75 (28.7)

n Sample size, SD Standard deviation

1462 Qual Life Res (2011) 20:1457–1468

123

44.4% had obtained a college degree. The control group

had slightly more men (n = 143 [54.8%]), and approxi-

mately thirty percent (28.7%) had obtained a college

degree. Both the insomnia and control groups were pre-

dominantly white (95.9 and 71%, respectively) and married

(67.2 and 60.9%, respectively).

Item-level results

The response frequency distributions for 20 of the 38 SFIS

items were negatively skewed (i.e., problematic floor

effects), indicating that sleep had only a slight to moderate

impact on daily functioning. Additionally, the majority of

Table 5 Item-level differences overall and between control and Insomnia

Item Overall Insomnia Control Effect

sizen Mean

(SD)

n Mean

(SD)

n Mean

(SD)

1 (1a): Tired during the day? 431 2.8 (1.0) 170 3.6 (0.7) 261 2.3 (0.9) 1.7

2 (1b): Exhausted during the day? 430 2.0 (1.0) 169 2.6 (1.0) 261 1.6 (0.8) 1.2

3 (1c): Sleepy during the day? 431 2.4 (1.0) 170 2.9 (0.9) 261 2.0 (0.8) 1.0

4 (1d): Like you had less energy during the day? 432 2.6 (1.1) 171 3.5 (0.8) 261 2.0 (0.9) 1.7

5 (2a): Motivation to do usual daily activities? 431 2.1 (1.1) 171 2.9 (0.9) 260 1.7 (0.9) 1.3

6 (2b): Motivation to care for yourself? 432 1.5 (0.9) 171 1.8 (1.0) 261 1.3 (0.7) 0.6

7 (3a): Difficult to focus on what you were doing? 431 1.9 (1.0) 171 2.6 (0.9) 260 1.5 (0.8) 1.3

8 (3b): Difficult to stay alert? 432 1.9 (1.0) 171 2.6 (0.9) 261 1.5 (0.7) 1.3

9 (3c): Difficult to remember things? 431 2.1 (1.1) 170 2.8 (1.0) 261 1.6 (0.8) 1.4

10 (3d): Difficult to make decisions? 432 1.7 (1.0) 171 2.4 (1.0) 261 1.3 (0.7) 1.3

11 (4): How good was the work you did? 431 2.9 (0.6) 171 3.3 (0.6) 260 3.0 (0.5) 0.6

12 (5a): How often do you put off doing things? 431 2.4 (1.0) 171 3.0 (0.9) 260 2.0 (0.9) 1.1

13 (5b): How often do you take longer to finish things? 432 2.2 (1.1) 171 3.1 (1.0) 261 1.7 (0.8) 1.6

14 (5c): How often do you have to force yourself to start things? 432 2.4 (1.2) 171 3.3 (0.9) 261 1.8 (0.9) 1.6

15 (5d): How often do you have to force yourself to finish things? 431 2.3 (1.2) 171 3.1 (1.0) 260 1.8 (0.9) 1.4

16 (5e): How often do you do just enough work to get by? 431 2.1 (1.0) 171 2.7 (1.0) 260 1.8 (0.9) 1.0

17 (5f): How often do you take time off from school/work? 431 1.7 (0.9) 170 2.1 (1.0) 261 1.5 (0.8) 0.7

18 (5g): How often do miss appointments? 432 1.2 (0.5) 171 1.3 (0.6) 261 1.1 (0.3) 0.4

19 (5h): How often do you get to school/work late? 430 1.6 (0.9) 171 1.9 (1.1) 259 1.3 (0.7) 0.7

20 (5i): How often do you doze off while working? 429 1.4 (0.7) 170 1.6 (0.9) 259 1.2 (0.6) 0.5

21 (5j): How often do you forget what you were doing while working? 432 1.6 (0.8) 171 2.1 (1.0) 261 1.2 (0.5) 1.3

22 (5k): How often do you make mistakes while doing something you had to do? 430 1.7 (0.8) 169 2.2 (0.8) 261 1.4 (0.6) 1.2

23 (6a): Negatively affected your enjoyment of social activities? 432 1.9 (1.0) 171 2.6 (1.0) 261 1.5 (0.8) 1.3

24 (6b): Negatively affected your relationships? 432 1.8 (1.0) 171 2.4 (1.0) 261 1.4 (0.8) 1.1

25 (6c): Negatively affected your interest in intimacy? 426 2.0 (1.2) 168 2.8 (1.3) 258 1.5 (0.8) 1.3

26 (7a): How often have you turned down or canceled plans? 432 1.6 (0.9) 171 2.1 (1.0) 261 1.2 (0.6) 1.1

27 (7b): How often have you wanted to be left alone? 432 2.2 (1.1) 171 2.9 (1.1) 261 1.8 (0.9) 1.1

28 (8a): How often have you felt irritable? 432 2.1 (0.9) 171 2.7 (0.9) 261 1.8 (0.8) 1.1

29 (8b): How often have you felt frustrated? 432 2.1 (1.0) 171 2.7 (0.9) 261 1.7 (0.8) 1.2

30 (8c): How often have you felt down? 431 1.8 (1.0) 170 2.4 (1.0) 261 1.5 (0.7) 1.1

31 (8d): How often have you felt anxious? 430 1.8 (1.0) 170 2.3 (1.0) 260 1.4 (0.8) 1.0

32 (8e): How often have you felt stressed? 432 2.3 (1.1) 171 2.9 (1.0) 261 1.8 (1.0) 1.2

33 (8f): How often have you felt overwhelmed? 431 2.0 (1.1) 170 2.7 (1.2) 261 1.6 (0.9) 1.1

34 (9a): How frustrated by lack of energy? 432 2.4 (1.2) 171 3.3 (1.0) 261 1.7 (0.9) 1.6

35 (9b): How frustrated by many mistakes you made? 431 1.8 (0.9) 170 2.4 (1.1) 261 1.4 (0.6) 1.3

36 (9c): How frustrated by things you would usually ignore? 431 1.9 (1.0) 171 2.6 (1.1) 260 1.5 (0.7) 1.2

37 (10a): How bothered by getting less done than planned? 431 2.4 (1.2) 170 3.3 (1.1) 261 1.8 (0.9) 1.5

38 (10b): How bothered by trouble focusing on what you were doing? 431 2.1 (1.1) 170 2.9 (1.1) 261 1.5 (0.8) 1.6

n Sample size, SD Standard deviation

Qual Life Res (2011) 20:1457–1468 1463

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items exhibited these floor effects in the control group, in

agreement with a lack of sleep problems in that group.

However, only four items (items 6, 18, 19, and 20) dem-

onstrated floor effects in both the insomnia and control

groups, indicating that most items did allow for an ade-

quate range of severity for the insomnia group. In addition

to the evaluation for floor and ceiling effects, items with

response categories that were never endorsed, indicating a

restricted response scale range, were flagged. This anomaly

occurred for three items: items 11 (Quality of work), 18

(Missed appointments), and 22 (Mistakes). For item 11,

over 90% of respondents endorsed only two categories

(i.e., categories 3 and 4); while for items 18 and 22, the

highest categories were never endorsed.

Displayed in Table 5 are the item-level descriptive sta-

tistics for the 38 SFIS items across the entire sample and by

insomnia and control groups, as well as the item-level

effect size estimates between the two groups (i.e., Cohen’s

d). In agreement with the item-level response frequency

distributions, the SFIS item-level means are lower than 3.0

and have a standard deviation of approximately 1, reflect-

ing that sleep, in general, had a mild impact on daily

functioning for the participants. The means for the whole

sample ranged from 2.9 for item 11 (SD = 0.6; Quality of

work) to a low of 1.2 for item 18 (SD = 0.5; Missed

appointments).

Effect sizes that are positive and greater than 1 indicate

that the insomnia group reported greater impact of sleep on

functioning than the control group. Overall, item-level

SFIS effect sizes can be characterized as large, indicating

that the majority of items are capable of distinguishing

between the two groups. Moderate effects were observed

for items 6 (d = 0.59; Care for self), 11 (d = 0.62; Quality

of work), and 20 (d = 0.50; Doze off); while small

effects were observed for item 18 (d = 0.38; Missed

appointments).

Correlational analyses

Of the 703 possible item pairs that are correlated, 25 pairs

of items have inter-correlations that exceed 0.80. Stem and

content overlap between these 25 item pairs are likely

causing redundancies, with multiple items assessing func-

tioning such as physical, cognitive, performing daily

activity, social, and sexual.

The majority of item-total correlations exceeded 0.70.

Item-total correlations less than or equal to |0.40| in either

the entire sample, or by group, were flagged as potential

candidates for removal. The low item-total correlations are

0.30 for item 3 (Feeling sleepy) in the insomnia group,

-0.39 for item 11 (Quality of work) across the whole

sample (-0.19 for control and -0.36 for insomnia), 0.40

for item 18 (Missed appointments) in the control group,

and 0.39 for item 20 (Doze off) across the whole sample

(0.35 for control and 0.31 for insomnia).

Item reduction

Based on the item-level descriptive statistics (e.g., floor

effects, restricted range, effect sizes, and the correlational

analyses), item reduction was performed and 12 out of 38

draft SFIS items were eliminated, yielding a final 26-item

version (SFIS). The deleted items included the following:

item 3 (Feeling sleepy) has low item-total correlation in the

insomnia group; item 6 (Grooming) and item 19 (Lateness)

exhibit floor effects in the whole sample and in the

insomnia and control groups; item 11 (Work quality), the

only reverse-scored item, has low item-total correlation and

a restricted response range across the whole sample and in

the two groups; item 20 (Doze off) has low item-total

correlation and exhibits floor effects across the whole

sample and in the two groups; while item 18 (Missed

appointments) and item 22 (Mistakes) have low item-total

correlation, exhibit floor effects, and have a restricted

response range across the whole sample and in the two

groups. In addition, items 34 through 38 were removed

because they may be measuring the impact of lack of

functioning rather than a lack of functioning due to sleep.

Further item reduction, through selection of one item from

each highly correlated item pair, was not considered at this

time.

Exploratory factor analysis

Eigenvalues from the principal components analysis were

examined to inform the number of dimensions underlying

the proposed 26-item SFIS. For the 26-item SFIS, the

first eigenvalue was 14.9, accounting for nearly 57% of

the variance. The second eigenvalue, 1.4, accounted for

an additional 5.3% of the variance, and the remaining 24

eigenvalues ranged from 1.2 to 0.09 (variance explained

from 4.7 to 0.4%). To assess the strength of the rela-

tionship between each of the 26 items and an underlying

factor, a one-factor EFA model was considered. Table 6

displays the factor loadings for the 26 items (all strong

and greater than 0.70). Considering the strong factor

loadings and that the addition of more factors would

explain a negligible amount of variance, the parsimoni-

ous one-factor model is recommended for the 26-item

SFIS.

The results were very similar for the EFA based on data

from the insomnia group alone. The first eigenvalue was

10.9, accounting for approximately 42% of variance; the

second was 2.2 (8.3%); the third was 1.6 (6.2%); and the

remaining eigenvalues ranged from 1.35 (5.2%) to 0.13

1464 Qual Life Res (2011) 20:1457–1468

123

(0.05%). The one-factor solution yielded factor loadings all

greater than 0.40.

Item response theory

According to the results of the EFA, the SFIS was ade-

quately represented by a unidimensional construct; there-

fore, all 26 items were considered in a graded response IRT

model. Results of the IRT analysis suggested that the SFIS

is more precise in the upper portion of the scale (i.e., the

majority of the item thresholds were in the positive region

of the construct), where comparisons among patients with

decreased functioning would most likely occur. The item

slope parameter estimates range from 1.38 to 3.42, with all

26 items being strongly and similarly related to the

underlying construct, a desirable property when sum-

scoring is applied.

Scale-level analyses

The descriptive statistics for the 26-item SFIS summary

measure and the other measures (i.e., ESS, FOSQ, ISI, MOS-

Sleep, WPAI–GH, and PSQI) used to explore content

validity are presented in Electronic supplementary material

across the entire sample and by group. Additionally, effect

size estimates between the nsomnia and control groups (i.e.,

Cohen’s d) are included. Given the EFA and IRT results,

SFIS scores were computed as sums across component items

with a possible range from 26 to 130. Higher SFIS scores

reflect greater impact of sleep on daily functioning. As

expected, the insomnia group scored higher on the SFIS than

the control group, 70.6 (SD = 16.4) versus 42.3 (SD =

13.8), with an effect size of 2.05. Effect sizes were moderate

to large and in the anticipated direction for the specific

instrument (i.e., insomnia group has greater dysfunction).

Reliability and construct validity

The internal consistency reliability of the 26-item SFIS is

very high at 0.97 for the entire sample and 0.95 for the

insomnia group. The construct validity correlations are

shown in Table 7. All correlations were statistically sig-

nificant. Strong correlations were found between the

26-item SFIS and the ISI Total Composite (r = 0.82), the

PSQI Overall Composite (r = 0.78), the FOSQ Activity

subscale (r = -0.76), the MOS-Sleep Problems Index I

and II (r = 0.74 and 0.76, respectively), and the PSQI

subscales Sleep Dysfunction and Subjective Sleep Quality

(r = 0.75 and 0.70, respectively). Moderately strong cor-

relations were observed between the SFIS and the FOSQ

Composite (r = -0.69), the WPAI–GH Activities

Impairment (r = 0.68), the two MOS subscales Sleep

Disturbance (r = 0.68) and Somnolence (r = 0.61), and

the PSQI Sleep Disturbance subscale (r = 0.63). The

remaining measures correlated moderately highly, ranging

from -0.56 (FOSQ Productive) to -0.42 (FOSQ Social).

Modest correlations were observed between the SFIS and

WPAI–GH Work Missed due to health (r = 0.31), and the

two MOS subscales pertaining to snoring and shortness of

breath (r = 0.26 and 0.27, respectively).

Consistent with expectations, the FOSQ Overall Com-

posite and ISI were strongly correlated with the SFIS, as

were the PSQI Composite and the MOS-Sleep Problems

indices. The weakest correlations were observed between

the SFIS and the MOS subscales that referred to snoring

and breathing.

Known-groups validity

SFIS scores were significantly higher (i.e., poorer func-

tion) in the insomnia group than the control group,

Table 6 26-Item SFIS EFA factor loadings

Item Factor 1

1 (1a): Tired during the day? 0.79

2 (1b): Exhausted during the day? 0.77

4 (1d): Like you had less energy during the day? 0.82

5 (2a): Motivation to do usual daily activities? 0.83

7 (3a): Difficult to focus on what you were doing? 0.84

8 (3b): Difficult to stay alert? 0.82

9 (3c): Difficult to remember things? 0.80

10 (3d): Difficult to make decisions? 0.83

12 (5a): How often do you put off doing things? 0.80

13 (5b): How often do you take longer to finish things? 0.85

14 (5c): How often do you have to force yourself

to start things?

0.86

15 (5d): How often do you have to force yourself

to finish things?

0.88

16 (5e): How often do you do just enough work to get by? 0.72

17 (5f): How often do you take time off from school/work? 0.64

21 (5j): How often do you forget what you were doing

while working?

0.80

22 (5k): How often do you make mistakes while

doing something you had to do?

0.79

23 (6a): Negatively affected your enjoyment

of social activities?

0.81

24 (6b): Negatively affected your relationships? 0.72

25 (6c): Negatively affected your interest in intimacy? 0.81

26 (7a): How often have you turned down or canceled

plans?

0.78

27 (7b): How often have you wanted to be left alone? 0.81

28 (8a): How often have you felt irritable? 0.86

29 (8b): How often have you felt frustrated? 0.81

30 (8c): How often have you felt down? 0.77

31 (8d): How often have you felt anxious? 0.82

32 (8e): How often have you felt stressed? 0.82

Qual Life Res (2011) 20:1457–1468 1465

123

t(430) = 19.36, P \ 0.0001, yielding an effect size of 2.05

(insomnia: mean = 70.59 and control: mean = 42.27,

SD = 13.79). Additionally, as anticipated, the SFIS scores

were significantly lower for those who reported an adequate

amount of sleep per night verses those who did not,

t(429) = -12.18, P \ 0.001 (effect size = -1.04, at least

7 h: mean = 42.61 and less than 7 h: mean = 63.24,

SD = 19.9) and for those with higher, hence, poorer PSQI

Sleep Quality scores, t(430) = 17.06, P \ 0.0001 (effect

size = 2.33, higher than 5 PSQI: mean = 65.01 and lower:

mean = 39.06, SD = 11.15).

Discussion

Patient focus groups, the published literature, and an

advisory panel generated the item content for the SFIS, and

information from cognitive debriefing interviews was used

to refine the draft questionnaire. Results from the psycho-

metric analyses of 432 patients were used to finalize the

item content and to evaluate the psychometric properties of

the SFIS. Based on the item-level evaluations (i.e., fre-

quencies, descriptives, effect sizes, and item correlations),

12 items were eliminated from the SFIS, and psychometric

analyses were performed on the 26-item SFIS scale.

Results of the principal components and factor analyses

provided overwhelming evidence that the SFIS is

unidimensional.

The factor analyses results are a bit surprising because

review of the instrument’s content, including concepts or

aspects addressed by groups of items, suggested that these

items might form separate, interpretable factors, or SFIS

subscales. The SFIS was developed to address seven

aspects related to the functional impact of insomnia: (1)

mood; (2) tiredness/energy; (3) memory/concentration; (4)

motivation; (5) daily performance; (6) social interaction;

and (7) sexual functioning. While the computation of

subscales could be considered, the single-factor scoring

structure of the SFIS-revised was corroborated by the

moderate to strong inter-item correlations and additional

IRT analyses, suggesting that computation of a total score

is most appropriate.

The results of the factor analyses, as well as the corre-

lational and IRT analytic results, support the use of con-

ventional unit-weighting of items for scoring the SFIS.

When construct validity correlations were evaluated, the

results were generally positive, providing satisfactory

preliminary support for the convergent construct validity of

the SFIS. The SFIS composite was most highly correlated

with the ISI, the PSQI Overall Composite, FOSQ Com-

posite and Activity subscale, the MOS-Sleep Problems

Index I and II, and the two PSQI subscales (Sleep Dys-

function and Subjective Sleep Quality). Moderately high

correlations were observed with the ESS total composite,

the remaining FOSQ subscales (Vigilance, Intimacy, Pro-

ductive, and Social) and PSQI subscales (Sleep Latency,

Duration, Efficiency, Disturbance, and Medication Usage).

The magnitude of the correlations indicates that the SFIS

composite correlates reasonably well with the other mea-

sures, but is not redundant with them.

The pattern of divergent correlations regarding reference

period was not identical to what was hypothesized,

reflecting that the impact of sleep disruption on functioning

was relatively stable for the participants over the assess-

ment period. As anticipated, the weakest correlations were

observed between the SFIS composite and the MOS

Table 7 Pearson validity correlations for the 26-item SFIS

(N = 272–432)

Scale 26-item

SFIS

ESS Total composite 0.46****

FOSQ: Activity -0.76****

FOSQ: Vigilance -0.54****

FOSQ: Intimacy -0.45****

FOSQ: Productive -0.56****

FOSQ: Social -0.42****

FOSQ: Composite -0.69****

ISI Total 0.82****

MOS: Sleep disturbance scale 0.68****

MOS: Snoring scale 0.26****

MOS: Short of breath scale 0.27****

MOS: Sleep adequacy -0.46****

MOS: Somnolence scale 0.61****

MOS: Sleep problems index I 0.74****

MOS: Sleep problems index II 0.76****

MOS: Sleep quantity (Raw Score) -0.48****

MOS: Optimal sleep scale -0.50****

WPAI–GH: Pct work time missed due to health 0.31****

WPAI–GH: Pct overall work impairment due to health 0.56****

WPAI–GH: Pct impaired while working due to health 0.55****

WPAI–GH: Pct other activities impaired due to health 0.68****

PSQI: Subjective sleep quality 0.70****

PSQI: Sleep latency 0.60****

PSQI: Sleep duration 0.47****

PSQI: Sleep efficiency 0.54****

PSQI: Sleep disturbance 0.63****

PSQI: Sleep medication usage 0.57****

PSQI: Sleep dysfunction 0.75****

PSQI: Overall composite 0.78****

**** P \ 0.0001

ESS Epworth Sleepiness Scale, FOSQ Functional Outcomes of Sleep

Questionnaire, ISI Insomnia Severity Index, MOS Medical Outcomes

Study, PSQI Pittsburgh Sleep Quality Index, SFIS Sleep Functional

Impact Scale, WPAI–GH Work Productivity and Activity Impair-

ment–General Health Questionnaire

1466 Qual Life Res (2011) 20:1457–1468

123

subscales that referred to disruption of sleep from snoring

and breathing.

The known-groups validity of the SFIS composite was

very strong. Participants in the insomnia group had higher

SFIS scores than those in the control group, reflecting

greater disturbance on functioning due to sleep deficiency.

Additionally, the group reporting adequate sleep had lower

SFIS scores and the group reporting poorer sleep quality on

the PSQI had higher SFIS scores. All three comparisons

were statistically significant at P \ 0.001.

Finally, the effect size of the difference in SFIS between

the insomnia and control groups was quite large (Cohen’s

d = 2.05), indicating that the SFIS was very capable of

discriminating between these two groups. Comparable

effect sizes of the validating instruments were also notably

large. The maximum was observed for the ISI total and

PSQI composite, both at approximately 4.0. Interestingly,

these two instruments had the strongest correlation with the

SFIS (r & 0.80) despite the longer reference periods and

different content domains. The MOS-Sleep Problems Index

I and II had somewhat higher effect sizes than the SFIS (at

2.4 and 2.7, respectively). The FOSQ subscales and com-

posite, ESS total, and WPAI–GH subscales were lower

than the SFIS. Selecting instruments based solely on the

magnitude of effect sizes alone is not recommended. The

content domain and reference periods are notably different;

while the SFIS, FOSQ, WPAI–GH, and ESS focus on the

impact of sleep on functioning, and the quality of sleep is a

major content domain for the ISI, PSQI, and MOS.

Recommendations

The 26-item SFIS was evaluated and exhibited satisfactory

psychometric properties. The 26 items preserve the aspects

covered by the draft 38-item SFIS: mood, tiredness/energy,

memory/concentration, motivation, daily performance,

social interaction, and sexual functioning. The results of

the evaluation also indicate that the 26-item SFIS scale is

extremely reliable and contains additional pairs of items

that are highly related. At this point, highly correlated item

pairs were retained because the present analyses provide

evidence that including them does not detrimentally impact

the scale. With respect to the scoring of the SFIS com-

posite, the 26 items should be summed, yielding a score

that can range from 26 to 130. Higher SFIS scores reflect

worse outcomes such that experiencing sleep difficulties

(e.g., difficulty initiating or maintaining sleep, nonrestor-

ative sleep) severely affects functioning. In cases where

item-level SFIS data are missing, we tentatively propose

that a respondent must answer at least 13 items or the SFIS

score should be set to missing.

Future studies might focus on further item reduction to

reduce administration time or explore potential differences

related to mode of administration. Gathering input from

patients (in a qualitative study) could be helpful in this

regard, both to identify the optimal items within the

redundant pairs and to ensure that the content validity of

the SFIS is preserved following item reduction or change in

mode. However, the current work does provide evidence to

support the SFIS as a psychometrically sound measure

targeting the impact of insomnia on patient functioning.

Administered along with a sleep diary, this instrument has

the ability to provide a more comprehensive assessment of

treatment response in clinical studies.

Acknowledgments We would like to acknowledge Mapi Values for

their work in conceptualizing and developing the SFIS, as well as

Knowledge Networks for the use of the KnowledgePanel� and for

administering the questionnaire. We thank the advisory panel par-

ticipants: Dr. Ruth Benca, University of Wisconsin; Dr. W. Vaughn

McCall, Wake Forest University; Dr. Martin Scharf, Tristate Sleep

Disorders Center; Dr. Jame K. Walsh, St. Luke’s Hospital; Dr. Gary

Zammit, Clinilabs, Inc.; and Dr. Phyllis Zee, Northwestern Univer-

sity. Funding for this study was provided by GlaxoSmithKline,

Research Triangle Park, NC.

Source(s) of Support GlaxoSmithKline

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