Quality in Health Care 1994;3:180-185

SF 36 health survey questionnaire: I. Reliability intwo patient based studies

Danny A Ruta, Mona I Abdalla, Andrew M Garratt, Angela Coutts, Ian T Russell

Department ofPublicHealth,University ofAberdeenDanny A Ruta, lecturerAndrewM Garratt,research fellowAngela Coutts, researchassistantHealth ServicesResearch Unit,University ofAberdeenMona I Abdalla, researchfellowIan T Russell, directorCorrespondence to:Dr D A Ruta,Department of Public HealthMedicine,Tayside Health Board,PO Box 75 Vernonholme,Riverside Drive,Dundee DD1 9NL

Accepted for publication30 September 1994

AbstractObjective-To assess the reliability of theSF 36 health survey questionnaire in twopatient populations.Design-Postal questionnaire followedup, if necessary, by two reminders at twoweek intervals. Retest questionnaires wereadministered postally at two weeks in thefirst study and at one week in the secondstudy.Setting-Outpatient clinics and fourtraining general practices in Grampianregion in the north east of Scotland(study 1); a gastroenterology outpatientclinic in Aberdeen Royal Hospitals Trust(study 2).Patients-1787 patients presenting withone of four conditions: low back pain,menorrhagia, suspected peptic ulcer, andvaricose veins and identified betweenMarch and June 1991 (study 1) and 573patients attending a gastroenterologyclinic in April 1993.Main measures-Assessment of internalconsistency reliability with Cronbach's a

coefficient and of test-retest reliabilitywith the Pearson correlation coefficientand confidence interval analysis.Results-In study 1, 1317 of 1746 (75-4%)correctly identified patients entered thestudy and in study 2, 549 of 573 (95-8%).Both methods of assessing reliabilityproduced similar results for most ofthe SF 36 scales. The most conserva-tive estimates of reliability gave 95%confidence intervals for an individualpatient's score difference ranging from-19 to 19 for the scales measuring physicalfunctioning and general health percep-tions, to -65 7 to 65 7 for the scale measur-ing role limitations attributable toemotional problems. In a controlledclinical trial with sample sizes of 65patients in each group, statistically sig-nificant differences of 20 points can bedetected on all eight SF 36 scales.Conclusions-All eight scales of the SF 36questionnaire show high reliability whenused to monitor health in groups ofpatients, and at least four scales possessadequate reliability for use in managingindividual patients. Further studies are

required to test the feasibility ofimplementing the SF 36 and otheroutcome measures in routine clinicalpractice within the health service.(Quality in Health Care 1994;3:180-185)

IntroductionIn an era of escalating healthcare costs andscarce resources, purchasers and providers ofhealth care require information that will allowthem to "estimate as best they can the relationbetween medical interventions and healthoutcomes."' Only then will they be able toachieve their stated goals of efficient and highquality medical care. The kind of informationsystems required will need to incorporatemeasures of outcome that are valid; reliable;responsive to clinically significant changes inhealth over time; and, above all, quick and easyto administer in a routine clinical setting. Fewoutcome measures currently available forroutine use satisfy all these criteria.2The short form 36 (SF 36) health survey

questionnaire is a shortened version of abattery of 149 health status questions used inthe RAND Corporation study of healthinsurance in the United States3 4 and wasdeveloped as a potential tool for monitoringpatient outcomes in a busy clinical setting. Thequestions were selected to produce aquestionnaire that could be completed inunder 10 minutes while retaining the valid-ity and reliability of the longer parentquestionnaire. The SF 36 questionnairemeasures three aspects of health: functionalstatus, wellbeing, and overall evaluation ofhealth using eight separate scales (table 1). Theresponses to the questions on each scale aresummed to provide eight scores between 0 and100. Because it is a general measure, thequestionnaire may be used to compare healthstatus both among patients with the samecondition and between patients with differentconditions. It may also be administered togeneral populations to see how a particular

Table 1 SF 36 health survey questionnaire scales

No ofitems

I Functional status(a) Physical functioning 10(b) Social functioning 2(c) Role limitations attributable to physical 4

problems(d) Role limitations attributable to emotional 3

problemsII Wellbeing

(a) Mental health 5(b) Energy and fatigue 4(c) Pain 2

III Overall evaluation of health(a) General health perception 5

Total 35*

*The 36th question, which asks respondents to compare theirhealth now with that one year ago, is not included within theseeight scales.

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condition causes health to depart from a"healthy standard." Providing that the criteriaof validity, reliability, and responsiveness arefully satisfied, the SF 36 questionnaire, as partof a routine outcomes monitoring system, hasenermous potential for bringing aboutimprovements in health care. For example, inthe context of resource allocation, it may helppurchasers to make difficult rationing decisionssuch that priority is given to those medicalconditions for which treatment is of provenbenefit (as in the Oregon experiment in theUnited States5); it may facilitate audit ofclinical practice' and evaluation ofnew medicalinterventions in clinical trials7; and it mayinform the process of clinical decision makingin managing individual patients by providingimmediate feedback on patient functioningand wellbeing during the doctor-patientconsultation.8

Strong evidence for the clinical and psycho-metric validity of the SF 36 questionnaire inpatient populations was provided by two recentstudies in the United States9 10 and a study inthe United Kingdom." In the UnitedKingdom study, for example, the SF 36 ques-tionnaire was able to show appreciable differ-ences in self reported health between thegeneral population and patients with fourcommon conditions." This included highlysignificant differences for patients with varicoseveins, a condition for which treatment has beenaccorded low priority by some purchasers ofhealth care in the United Kingdom.'2Two approaches can be taken to test

reliability: internal consistency, where similarquestions within a scale are assessed for theextent to which they are correlated with eachother and with the overall total score,'3 andtest-retest, where two measurements separatedby some interval of time are assessed fordifferences. 13 Controversy exists over therelative merits of both procedures.'3 '4 Test-retest reliability has been put forward as themore rigorous approach when the instrumentunder investigation is to be used as anevaluative measure to detect changes in healthstatus over time.'4 A major problem withtest-retest, however, is its tendency tounderestimate reliability if true change hasoccurred'3 - that is, if patients actually getworse or improve between measurements.Although the internal consistency of the eightSF 36 scales has been well documented in bothpatient and general populations (C AMcHorney et al, unpublished data),8 115-18only one study, which used a generalpopulation sample, reported the test-retestreliability.' If the SF 36 questionnaire is to beused to assess changes in health status overtime its test-retest reliability should bedemonstrated in patient populations.We report the results of two studies in which

both internal consistency and test-retestreliability were assessed, the first in more than1700 patients presenting with four commonconditions (low back pain, suspected pepticulcer, menorrhagia, and varicose veins) and thesecond in a study of just over 570 patientsattending a gastroenterology outpatient clinic.

MethodsSAMPLING AND DATA COLLECTION

Study 1Between March and June 1991 we identifiedpatients in Grampian region presenting withone of four common conditions: low back pain,menorrhagia, suspected peptic ulcer, andvaricose veins. The patients were identified inone of two ways: from all referral letters tooutpatient departments in Grampian and bygeneral practitioners from four large trainingpracices in Grampian, these patients beingincluded only if the general practitioner didnot refer them to a specialist duringthe recruitment period of the study. Aquestionnaire including an anglicised versionof the SF 3616 questionnaire' and sociodemo-graphic questions was sent to the patients ingeneral practice within two weeks of theirinitial consultation and to the referred patientsbefore their first outpatient appointment.Patients not wishing to take part in the studywere asked to return their questionnaireuncompleted. Reminders were sent to non-responders after two weeks and again after fourweeks. In order to assess test-retest reliabilitya sample of patients returning a questionnairewas sent a retest questionnaire after two weeks.This included an additional question whichasked, "Since you last completed a question-naire, would you say that your health hasimproved, got worse, or stayed the same?"

Study 2All patients attending the gastroenterologyoutpatient clinic in Aberdeen Royal HospitalsTrust during April 1993 were asked tocomplete a similar questionnaire, containingthe SF 36 instrument, in the clinic waiting areabefore seeing the doctor. In order to assess test-retest reliability, a subsample of patients withulcerative colitis was sent a retest questionnaireone week later, including three additionalquestions asking whether their physical,mental, or general health had improved, gotworse, or stayed the same since they had lastcompleted a questionnaire.

STATISTICAL ANALYSIS

We used two methods to assess reliability:internal consistency and test-retest reliability.Internal consistency was assessed usingCronbach's a(,19 which measures the cor-relations between questions that make up ascale at a single point in time. Alphacoefficients were calculated for each of theeight SF 36 scales based on responses to theinitial questionnaire. An ax coefficientexceeding 0-5 is considered acceptable whencomparing groups of patients,20 althoughcoefficients exceeding 0-9 have beenrecommended when making comparisonsbetween individual patients or assessingchange in scores in an individual patient overtime.2' 22 In order to demonstrate test-retestreliability it is necessary to show consistencybetween questionnaires for those patientsreporting no change in health. The differencebetween the scores of these patients for bothquestionnaires was calculated, and the

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standard deviations of those differences wereused to calculate 95% confidence intervals forthe differences in SF36 score for individualpatients."To make a thorough comparison of the two

methods of assessing reliability the axcoefficients could be used to construct 95%confidence intervals of the differences. Themeasurement of any patient's true score is arandom variable with a variance which is equalto the product of the variance of the scoresand one minus the reliability coefficient.13Therefore when two independent measure-

ments are made for all patients who reportedno changes in their health, at two differentpoints in time, 95% confidence intervals oftheir individual measurement differences couldbe constructed using statistical first principleanalysis.23 The results of both methods ofreliability assessment and the limits of thecorresponding 95% confidence intervals for an

individual's SF 36 score differences are pre-sented separately for the two study popula-tions.

Finally, to illustrate how reliability affectsthe precision of the SF 36 questionnaire - thatis, its statistical power to detect clinicallyimportant differences in health status betweendifferent groups of patients, sample sizeestimates were calculated.24 Estimates ofsample sizes required to detect small to largegroup differences in mean SF 36 scores, asdefined by Ware and colleagues,22 were

calculated for two kinds of study design: a

randomised controlled clinical trial withcomparisons between repeated SF 36assessments over time and a before and aftercomparison of SF 36 scores in a single patientgroup.

ResultsRESPONSE RATE

In study 1 a total of 1787 patients were

identified, 800 by their general practitionersand 987 from referral letters, of whom 236failed to respond and 193 refused to take part;41 questionnaires were returned by the postoffice. Thus 1317 of 1746 correctly identifiedpatients took part in the study, giving a finalresponse rate of 754%. Their ages rangedfrom 16 to 86 years (mean 42-7 years), and 870(66-1%) were female. Compared with thosereturning a questionnaire, non-responderswere significantly younger (mean age 39-9

years; p < 0 01) but did not differ significantlyin terms of sex, clinical condition, or source. Atotal of 775 retest questionnaires were postedto patients. Of the 710 (91 60/0) returnedcompleted, 414 (58.30 o) reported no change inhealth status since returning the previousquestionnaire.

In study 2, 573 patients were identified inthe clinic, of whom, 549 (95 8' /0) patientsreturned a questionnaire. Their ages rangedfrom 12 to 86 years (mean 48-1 years), and 344(62 7%) were female. Retest questionnaireswere posted to 70 patients with ulcerativecolitis. Of the 63 (90%) returned completed,48 (53%0) reported no change in physical,mental, or general health since completing thelast questionnaire.

STATISTICAL ANALYSIS

In study 1 (the general practice and referredpatients) estimates of reliability ranged from0-80 (social functioning) to 0-92 (physicalfunctioning), and from 0-66 (role limitationsattributable to emotional problems) to 0 93(physical functioning) with the internalconsistency and test-retest methods re-spectively (table 2). The limits of the 950%0confidence intervals for an individual patient'sSF 36 score differences ranged from +20 3(physical functioning) to +45 2 (rolelimitations - emotional problems) based on theCronbach's ax and from + 19 (physicalfunctioning and general health perceptions) to+65 7 (role limitations - emotional problems)using the standard deviation of scoredifferences from the test-rest analysis. Thehighest discrepancy between the two methodswas in their reliability estimates for rolelimitations attributable to physical problemsand role limitations attributable to emotionalproblems.

In study 2 (the gastroenterology clinicpatients), correlation coefficients ranged from0 80 (social functioning) to 0.93 (physicalfunctioning), and from 0 79 (role limitationsattributable to physical problems) to 0 94(physical functioning) using the internalconsistency and test-retest methods re-spectively (table 3). The limits of the 95%confidence intervals for an individual patient'sSF 36 score differences ranged from +20 7(physical functioning) to + 41 1 (rolelimitations - emotional problems) based on theCronbach's ca, and from + 12 0 (mental health)

Table 2 Internal consistency and test-retest reliability estimates for eight SF 36 scales in patients with low back pain, mnenonlhagia, u-spectedl pctic l/er,;and varicose veins

Scale Reliability estimates with internal consistency method Reliability estimates with test-retest methodan = 1286 minimum) (n = 414 niinimuni)

a coefficient Mean Standard Limits of95% Pearson Mean Standard I mmts f95f%SF 36 deviation confidence interval correlation SF 36 deviation of conlfdcn; intervalscore for individual coefficient score differecL t it divindul

SF 36 score difference SF 3616sCdifference di/f/erueLn

Physical functioning 0-92 70 7 26 0 ±20 3 0 93 0-96 9 71 1903Social functioning 0-80 70 2 26-5 +32 7 0 80 0-94 16 01 31 38Role limitations:

Physical 0-89 43-6 43-1 +39-6 0-76 4 91 29 06 56 96Emotional 0-86 57-6 43-6 +45-2 0-66 1 57 33-52 +65 70

Mentalhealth 0-86 65 1 199 +207 081 -041 11 55 92264Pain 0-86 50 3 26 8 ±27 7 0 82 1-07 1557 30 52Energyand fatigue 0-86 47 0 21 8 ±22-6 0 84 0-53 12907 23 66General health 0-83 62-9 22 5 ±25-7 0 88 2-0 9 71 19 03

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Table 3 Internal consistency and test-retest reliability estimates for eight SF 36 scales in patients attending gastroenterology outpatient clinic

Scale Reliability estimates with internal consistency method Reliability estimates with test-retest method(n = 544 minimum) (n = 47 minimum)

a coefficient Mean Standard Limits of 95% Pearson Mean Standard Limits of95%SF 36 deviation confidence interval correlation SF 36 deviation of confidence intervalscore for individual coefficient score difference for individual

SF 36 score difference SF 36 scoredifference difference

Physical functioning 0-93 74-8 28-2 ±20-7 0-94 0-20 8-86 ± 17-36Social functioning 0-80 76-0 27-8 +34-5 0-87 -0-16 12-35 ±24-21Role limitations:

Physical 0-89 62-3 43-1 ±38-7 0-79 3-90 23-50 ±46-06Emotional 0-87 73-6 41-1 ±41-1 0-81 0.0 21-74 ±42-61

Mental health 0-84 71-7 19-3 ±21-4 0-95 -0-15 6-11 +11-97Pain 0-86 67-0 27-1 ±28-1 0-81 -1-74 12-67 +24-80Energy and fatigue 0-88 54-1 23-1 ±22-2 0-92 -1-04 8-81 ±17-23General health 0-85 56-7 23-3 ±25-0 0-86 -0-38 11-00 ±21-56

to 46- 1 (role limitations - physical problems)using the standard deviation of scoredifferences from the test-retest analysis. Thehighest discrepancy between the two methodswas in their reliability estimates for rolelimitations attributable to physical problemsand for mental health.

Finally, the effect of different degrees ofreliability on study sample sizes required todetect differences in mean SF 36 scores for twotypes of study design was calculated. Thesample size estimates required to detect smallto large differences for a randomised controlledclinical trial with comparisons betweenrepeated SF 36 assessments over time areshown in table 4. To detect a large differenceof 20 points on all eight scales a sample size of64 is required. The sample sizes required todetect the same differences for a before andafter study design, with a comparison of SF 36scores in a single patient group are shown intable 5. To detect a large difference of 20points on all eight scales a sample size of 22 isrequired.

Table 4 Estimates of sample size required in each group to detect 2-20 point differences inchanges over time between two randomly selected patient groups

Scale No ofpoints difference

2 5 10 20

Physical functioning 2544 407 102 26Social functioning 2478 397 100 25Role limitations:

Physical 6408 1026 257 64Emotional 6185 990 248 62

Mental health 1405 225 57 14Pain 2563 410 103 26Energy and fatigue 1714 275 69 18General health 1816 291 73 19

Estimates assume a = 0-05, two tailed test, power = 0-80, standard deviation and the mostconservative estimates of reliability from the study of four common conditions.

Table 5 Estimates ofsample size required to detect 2-20 point differences over time in onepatient group

Scale No ofpoints difference2 5 10 20

Physical functioning 185 30 8 2Social functioning 503 81 20 5Role limitations:

Physical 1656 265 63 17Emotional 2202 353 88 22

Mental health 262 42 11 3Pain 476 76 19 5Energy and fatigue 286 46 12 3General health 185 30 8 2

Estimates assume a = 0-05, two tailed test, power = 0-80, and estimates of standard deviation ofthe difference taken from test-retest analysis in the study of four common conditions.

DiscussionThe internal consistency of the eight scalesmaking up the SF 36 questionnaire hasbeen reported in at least six publishedpapers.5 " '5-18 However, this form of reliabilitytesting relies on a single administration of thequestionnaire, which may lead to "anoptimistic interpretation of the true reliabilityof the test."'3 A formal assessment of SF 36questionnaire test-retest reliability was re-

ported in only one study,'6 in which re-spondents were selected at random from two

general practitioner lists and thus representedthe general population. In this paper we reporton a comprehensive assessment of SF 36reliability in two patient based studies.

In both studies the estimates of reliabilitybased on internal consistency were remarkablysimilar, and all were within the rangeof estimates reported elsewhere.5 '11'-18Appreciable differences between the test-retestand internal consistency methods ofassessment were seen in study 1, but only forthe two role limitation scales (in favour of theinternal consistency method), and in study 2for the mental health scale (in favour of thetest-retest method). For example, the limits ofthe 95%/o confidence intervals around an

individual patient's SF 36 score difference on

the mental health scale in study 2 were twiceas wide when generated using internalconsistency compared with the test-retestlimits. It is worth noting here that in study 2the time interval between administration of thequestionnaire was only one week, and patientswere included in the analysis only if they hadreported no change in physical, mental, or

general health. In study 1 the interval betweenadministrations was two weeks, and patientswere asked only whether their health hadchanged.

Despite the above mentioned differencesbetween the two methods of reliabilityassessment, both methods produced similarresults for most of the SF 36 scales. However,the accuracy of internal consistency estimatesare generally likely to be higher than that of thetest-retest method since, as in both of our

reported studies, estimates of internalconsistency are usually based on much largersamples. Test-retest requires the administra-tion of an additional questionnaire, and istherefore usually undertaken on a smaller sub-sample of the main survey.

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In summary, the widely held view thatinternal consistency, as a form of reliabilityassessment, may lead to "an optimisticinterpretation of the true reliability of thetest"" has not been consistently supported bythe results of our studies. These findings are inaccordance with research on reliability in testsof mental ability; Lord and Novick, forexample, found that test-retest methods mayoverestimate or underestimate reliability,depending on the time interval betweenadministrations, and concluded that "internalconsistency provides an accurate approxl-

mation to the reliability if the test underinvestigation is either reasonably hom-ogeneous, or of substantial length and notspeeded."25

Tables 4 and 5 were constructed from themost conservative reliability estimates fromstudy 1. The findings in these tables indicatethat all eight SF 36 scales show very high levelsof reliability when used to monitor healthstatus and health outcome in groups ofpatients, so that even with sample sizes ofaround 65 patients in each group in a

controlled clinical trial for example, statisticallysignificant differences of 20 points can bedetected on all eight scales. With sample sizesof only 30 patients in each group, statisticallysignificant differences of 20 points are

detectable on six of the eight scales. These datamay be used as a guide by those wishing toestimate sample size for their studies, but otherdesign factors may also need to be taken intoaccount. If the SF 36 questionnaire is used toassess health status and health outcome as partof individual patient management however,our findings suggest that some of the SF 36scales are not reliable enough to be of practicalvalue. For instance, if a patient obtains a score

of 50 on the scale measuring role limitationsattributable to emotional problems, from theresults obtained in our two studies, thatpatient's true score could be anywhere between9-91, and 15-85, based on test-retest resultsfrom studies 1 and 2 respectively. Individualscores obtained on the scale measuring rolelimitations attributable to physical problemsalso have wide confidence intervals. However,the reliability of scales measuring bodily painand social functioning is probably adequate fordetecting clinically important differencesbetween individual patients or betweensequential measurements on the same patient.For example, if a patient with an arthritic hipscored 10 on the SF 36 scale measuring bodilypain, after hip replacement that patient wouldneed a score of over 36 or 31 (based on resultsfrom studies 1 and 2 respectively) in order to

register a significant improvement at the50 o level. The remaining scales - physicalfunctioning, mental health, energy or fatigue,and general health - are sufficiently reliable tobe able to detect even smaller differences. Thesame arthritic patient scoring 10 on the SF 36physical functioning scale before surgery wouldneed to achieve a postoperative score of over26 or 25 (based on studies 1 and 2 respectively)to register a statistically significant improve-ment at the 5%0 level.

In conclusion, for a measure of health statusand health outcome to be suitable for routineuse in the NHS in a wide variety of clinicalsettings, it must provide information that isvalid, reliable, responsive to change, quick toadminister, and easy to collect. Our findingssuggest that the SF 36 questionnaire is reliableenough to be used for monitoring health ingroups of patients, and at least four of its scalespossess adequate reliability for use in managingindividual patients. In the accompanyingarticle (p 186)26 we have also been able to showthat the SF 36 questionnaire is responsive toclinical changes in health status over time.When this evidence is considered together withother published data on validity'1" 1 6 andreliability' 1 15 we believe it is sufficient toconclude that the SF 36 questionnaire resultsatisfies the requirements of a routine outcomemeasure. Further studies are now required totest the feasibility of implementing this andother outcome measures in routine clinicalpractice within the health service.

We thank the staff of Inverunie, Portlethen, Rubislasw Place, andWesthill practices, and the staff of the gastroenterology unit andoutpatient clinic at Woolmanhill for their help. We also thankJohn Masson, Jeremy Grimshaw, Dean Phillips, and JohannCoutts for the data collection and Elizabeth Russell for helpfulcomments. This research and the Health Services ResearchUnit are both funded by the Chief Scientist Office of theScottish Office Home and Health Department; however, theopinions expressed are those of the authors, not the SOHHD.

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