Research report (ONS) national Omnibus Survey (http://www.statistics.gov. uk) in 2008. This is a rolling face-to-face interview survey with adults aged 16+, living at home, based on a stratiﬁed random sample of postcodes across Britain; the response rate was 61% (n¼589). 3. QoL follow-up survey in 2007e2008, of people living at home in Britain, aged 65+ at baseline, who had responded to four ONS national Omnibus interview surveys. These were based on stratiﬁed random samples of postcodes across Britain during 1999/2000; response was 77% (n¼999) at baseline and 58% among survivors (n¼287) at 2007e2008 follow-up. The QoL follow-up survey is included here as the longitudinal design provided the opportunity to test the causal model of the OPQOL, as well as a willing sample for test-retest reliability assessment. Measures The OPQOL was administered in all three surveys. Prior to administration in the surveys reported here, the items in the OPQOL were pretested with 179 older people and three focus groups, reduced to 32-item and 35-item versions, and statistical tests of reliability and validity were applied. The CASP-1911 and the WHOQOL-OLD12 13 were administered in the two face-to- face interview surveys only; it would have been too cognitively burdensome to have included all three scales in the postal, self- administration mode. Supplementary Appendix 2 displays the OPQOL, summarises its development and brieﬂy summarises the CASP-19 and WHOQOL-OLD. Independent self-ratings of global QoL, and of its domains, were included in the questionnaire in order to distinguish between the constituents of, and inﬂuences on, QoL.14 Also included were standard sociodemographic items, self-rated active ageing, items measuring health and psychosocial circumstances.7 Ethnic status was measured using a standard item about ethnic identity in the UK. This would not necessarily be applicable to populations in other countries, because it reﬂects close connec- tions between New Commonwealth countries and ethnic minority groups in the UK.15 Statistical analysis Descriptive analyses included frequencies, c2 tests, and Spear- man’s r correlations. Tests of scale reliability were applied in order to assess the extent to which scale items measure the same construct, with freedom from random error (internal consis- tency). Reliability tests applied to the QoL scales included Cronbach’s a test of homogeneity. This is the strength of the association between each scale item and the full scale, item‑item and item‑total correlations. Test‑retest reliability of the stability of the newly developed OPQOL was assessed by mailing a second copy of the questionnaire to a random subsample of 50 follow-up QoL survey respondents, 4 weeks after return of the ﬁrst questionnaire (response rate: 76%/38). Criterion (concurrent) validity is the independent corrobora- tion that the scale is measuring what it intends to measure. This can only be measured by proxy with subjective measures, as there is no gold standard. Proxy variables used here included independent self-ratings of QoL overall and of QoL domains (health, social relationships, independence/control over life/ freedom, home and neighbourhood, psychological/emotional well-being, ﬁnancial circumstances, social and leisure activities). Construct (convergent and discriminant) validity requires corroboration that scales measure the underlying construct they purport to measure. This was tested by assessing Spearman’s r correlations between the QoL scales and similar variables (for 274 convergent validity that the scale should correlate with similar or hypothesised variables) and dissimilar variables (for discrim- inant validity that there should be low correlations between scales and variables not expected to be associated). Multiple regression was used to assess validity further by examining the ability of theoretically relevant variables to predict total QoL scores. A hierarchical approach was used, with independent variables entered in their theoretical order of importance. Statistical signiﬁcance was set at p<0.05. The variables entered did not correlate by more than 0.732; tests for multicollinearity were satisﬁed. Sociodemographic variables were entered to adjust for their effects. RESULTS Characteristics of samples Just over half of each sample comprised women (52%/207 Ethnibus, 55%/324 ONS, 54%/154 QoL follow-up). Whereas most Ethnibus respondents were aged 65<75 (91%/363), just over half of ONS Omnibus (55%/326), and less than a ﬁfth of QoL follow-up respondents (17%/47), were aged 65<75. Thirty- eight per cent (152) of the Ethnibus sample were Indian, 29% (117) were Pakistani, 22% (86) were Black Caribbean and 11% (45) were Chinese. Most, 94% (555) of the ONS Omnibus sample were white British; all QoL follow-up respondents were white British. In reﬂection of their younger age, more of the Ethnibus than other respondents were married or cohabiting (58%/230, 49%/285, 49%/138 respectively). Fewer Ethnibus than other respondents were home-owners (532%/208, 73%/ 429, 85%/239 respectively) and fewer lived alone (5%/19, 48%/ 286, 49%/137 respectively) (All differences were statistically signiﬁcant at least at p<0.01.) For detailed characteristics of the samples, see supplementary table 1. Distributions of samples on QoL scales Few, 12%/70, of the ONS Omnibus sample, compared with more, 45%/113 of the older QoL follow-up sample, and 73%/290 of the Ethnibus sample were in the lowest two OPQOL cate- gories (<119), indicating worse QoL (see supplementary table 2). The Ethnibus and ONS cross-sectional samples only were administered the CASP-19 and WHOQOL-OLD. Consistent with the OPQOL ﬁndings, 23%/94 of Ethnibus respondents were in the worst two CASP-19 categories (<29), compared with 8%/43 of ONS respondents; 25%/100 of the Ethnibus sample fell in the worst two WHOQOL-OLD categories, compared with 15%/80 of the ONS respondents (see supplementary tables 3 and 4). Further analyses by total QoL scores and ethnicity in the Ethnibus sample showed that 58% (26) of Chinese people scored a good QoL with the OPQOL, compared with 28% (33) of Pakistani, 20% (31) of Indian and 23% (31) of Black Caribbean people (c2 test 28.064, 2 degrees of freedom, p<0.001) (caution: smaller numbers). The CASP-19 and WHOQOL-OLD total scores showed no differences by ethnicity. Differences by ethnicity were not analysed in the other samples due to their low numbers in ethnic minority groups. Reliability The reliability criterion for item‑total correlations (the correla- tion of the item with the scale total with that item omitted) is that the item should correlate with the total scale by at least 0.20. With three exceptions, the 35 full OPQOL items met this criterion for all three samples (the exceptions were in the Ethnibus sample with items 10, 12 and 32; but as Cronbach’s a was not improved by their removal, and they performed well in validity tests, they were retained). Six of the 19 CASP items J Epidemiol Community Health 2011;65:273e280. doi:10.1136/jech.2009.087668