Lecture 5: Reliability and validity of scales

1. Describe the applications of the following types of measurement:

- Impairment, disability, handicap, quality of life, attitudes, behaviour

- Generic versus disease-specific health status and quality of life scales

2. Define the following terms, giving examples of each:

- Response bias

- Social desirability

3. In relation to scales, define the following terms:

- Test-retest reliability

- Inter-rater reliability

- Internal consistency

Scales

• Single- vs multi-item scales

• Items are intended to sample the content of the underlying construct

• Items summarized in various ways:– sum or average of responses to individual items– item weighting or other algorithm– profiles/sub-scale scores

3

International Classification of Impairments, Disabilities, and Handicaps (ICIDH)

• IMPAIRMENT:

– ...loss or abnormality of psychological, physiological, or anatomical structure or function.

• DISABILITY:

– ...restriction or lack (resulting from an impairment) of ability to perform an activity …

• HANDICAP:

– ...disadvantage... resulting from an impairment or disability, that limits or prevents the fulfillment of a role ….that is normal for that individual….

Quality of life (QoL)

• Definition – individuals’ perception of their position in life in the

context of the culture and value systems in which they live and in relation to their goals, expectations, standards, and concerns” (WHO QOL group, 1995)

• Domains– physical, psychological,level of independence, social

relationships, environment, and spirituality/religion/personal beliefs

Health-related quality of life (HRQoL)

• Dimensions of QoL related to health

• Related terms:– health status– functional status

• Usually includes:– physical health/function– mental health/function – social health/function

Selection of measures: Appropriateness

• Purpose:– describe health of population

– evaluate effects of interventions (change over time)

– compare groups at point in time

– predict outcomes

• Areas of function covered

• Level of health

• Generic/global or specific

Generic vs specific

• Generic– comparisons across populations and problems– robust and generalizable– measurement properties better understood

• Disease-specific– shorter– more relevant and appropriate– sensitive to change

Practical considerations

• Mode of administration– self-administered (in-person, mail)– interviewer (face-to-face, telephone)– informant or proxy

• Respondent burden

Example of single-item measure of HRQoL: the EuroQol

“thermometer”

• EITHER: visual analogue scale

• OR : Now, to help people say how good or bad their health state is, let’s say the best state you can imagine is 100 and the worst state you can imagine is 0.

• In your opinion, how good or bad is your health today - please use a number.

Example of Disability Scale: OARS ADL scale

• Measures basic and instrumental activities of daily living (ADL)

• 14 items: e.g., bathing, dressing, money management, house-cleaning

• Based on self-report and/or judgement• Response scale:

– Completely independent (2)– Needs some help (1)– Completely dependent (0)

Example of measure of health status/ HRQoL: SF-36

• Generic measure of health status• 36 items, self-report• Sample item: • Scoring:

– 8 specific sub-scales (e.g., physical function, mental health, vitality

– 2 component summary scores: physical and mental health

During the past 2 weeks, did you have any of the following problems with your work or other regular daily activities as a result of your physical health?

01 3b. Were limited in the kind of work or other activities

01 3a. Accomplished less than you would like

NOYES

How much bodily pain did you have during the past 2 weeks? Was there no pain, very mild pain… (etc)None 1Very mild 2 Mild 3Moderate 4Severe 5Very severe 6

Example of specific scale: Geriatric Depression Scale

• 15 or 30-item self-report scale

• Response options: yes/no

• Sample items:– Do you feel happy most of the time?– Do you feel that your life is empty?

Response bias

• Examples:– Recall– Acquiescence– Social desirability

• Factors affecting response bias:– Question wording/response scale– Characteristics of subjects: (age, education, etc)– Mode of data collection (questionnaire, interview,

telephone vs face-to-face)

Social desirability

• Tendency to give answers to questions that are perceived to be more socially desirable than the true answer

• Different from deliberate distortion (“faking good”)

• Depends on: – Individual characteristics (age, sex, cultural

background)

– Specific question

Social desirability

• Measures of social desirability (SD)– SD scales (e.g., Jackson SD scale, Crowne & Marlowe

SD scale)

– individual tendency to SD bias

• Prevention– phrasing of questions

– questionnaire mode

– training of interviewers

Reliability of scales

• Internal consistency

• Test-retest reliability

• Inter-rater and intra-rater reliability

Example: Delirium Index (DI)

• Delirium = acute confusional state• Characterized by acute onset and fluctuations• Risk factors:

– Predisposing: age, dementia, disability, comorbidity etc– Precipitating: infections, medications, environment

• DI: observer-rated measures of severity of 7 symptoms of delirium:– inattention, disorganized thinking– altered consciousness, disorientation– memory impairment, perceptual disturbances– psychomotor agitation or retardation

Administration and scoring• Administered by research assistant based on

patient observation

• Each symptom rated on 4-point scale:0 = absent

1 = mild

2 = moderate

3 = severe

Total score: range from 0 - 21

Evaluation of performance of DI

• What aspects should be evaluated?

• How?

Internal consistency

• Relevant to additive scales (that sum or average items)

• Split-half reliability:– correlation between scores on arbitrary half of

measure with scores on other half

• Coefficient alpha (Cronbach) – estimates split half correlation for all possible

combinations of dividing the scale

Example

• Internal consistency of Delirium Index scale to measure symptoms of delirium:– Cronbach’s alpha for entire scale:– ….without perceptual disturbance:

Test-retest reliability (stability)

• Scale is repeated – short-term

• for constructs that fluctuate, 2 weeks often used to reduce effects of memory and true change

– long-term• for constructs that should not fluctuate (e.g.,

personality traits)

• Some measure of variability vs stability of 2 scores is computed

Mean within-patient standard deviation in DI score during 1st week in hospital

0

0.5

1

1.5

2

2.5

3

Delirium+dementia (n=157)

Delirium (n=57)

Dementia (n=55)

Neither (n=41)

Inter- and intra-rater reliability

Inter-rater reliability

• For scales requiring rater skill, judgment

• 2 or more independent raters of same event

Intra-rater reliability

• Independent rating by same observer of same event

Measures of inter- and intra-rater reliability: continuous data

• Measures of correlation– Correlation graph (scatter diagram)– Correlation coefficients

• Measures of pairwise comparison

Correlation coefficients

• Pearson’s r– assesses linear association, not systematic

differences between 2 sets of observations– sensitive to range of values, especially outliers

• Spearman r– ordinal or rank order correlation– less influenced by outliers– doesn’t assess systematic differences

Correlation coefficients

• Intra-class correlation coefficient (ICC)– Estimate of proportion of total measurement

variability due to between-individuals (vs error variance)

– Equivalent to kappa and same range of values– Reflects true agreement, including systematic

differences – Affected by range of values - if less variation

between individuals, ICC will be lower

Inter-rater reliability

• Intraclass correlation coefficient (ICC):

n = 26 patients (39 pairs of ratings)

ICC = 0.98 (SD 0.06)

Examples for discussion

• What aspects of reliability should be measured for the following scales:– EuroQol VAS– SF-36– Geriatric Depression Scale