BMC Public Health 2006, 6:268 http://www.biomedcentral.com/1471-2458/6/268 residence) we also collected some limited socio-demo- graphic (marital status, education, own housing versus rental, occupational status) and disease related informa- tion (type of diabetes, duration of diabetes since the first diagnosis, medication for diabetes, complications of dia- betes). Statistical analysis We evaluated the psychometric characteristics of the Ara- bic translation of WHOQOL-BREF in the sample by means of internal reliability (Cronbach's alpha). We used the chi-square test for comparison of categorical variables and the t-test for the comparison of mean score values for the domains of WHOQOL-BREF between groups. Further, we performed multivariate analysis of variance to analyse the determinants of HRQOL in both groups. All inde- pendent variables were dichotomised. We followed stand- ard recommendations by Hosmer and Lemeshow for model-building strategies [21]. Since most of the variables had a significant interaction with the age and case-control status including them all jointly in the model would yield too many variables per observation. We therefore included interactions in two separate sets: interactions between case-control status and all other variables and interactions between age groups and all other variables. From each set we eliminated interactions which did not cross the significance level of 0.1. Finally, we included all interactions from the previous step in the model and elim- inated stepwise all terms which were not significant on the 0.05 level. Based on the final model we calculated the contribution of interactions to the HRQOL with Wald- type confidence intervals, comparing to a single reference category for all strata. All analyses were performed with SPSS® for Windows version 12, for the analysis of variance we used the GLM procedure and obtained the significance statistics from multivariate tests including all domains using the Pillai-Spur criterion. The choice of multivariate technique was motivated by a potential correlation of domain scores within the respondents and had the benefit of a homogenous set of independent variables in the models for all outcomes despite using variable selection. The interpretation of the results is based on a clinically meaningful difference of 5 points of the 0–100 scores. Results Psychometric characteristics of the WHOQOL-BREF in the study sample The internal reliability was very high for three domains and reasonably high for the fourth one. Cronbach's alpha for the physical health domain was 0.95, for the psycho- logical domain 0.94, for social relationships domain 0.6 and 0.85 for the environment domain. For the physical health, psychological and environment domains, the internal reliabilities could not be improved through removing any of the items. In the social relationships domain, the item related to sexual life satisfaction had a considerably lower mean than the remaining items: per- sonal relationships and support from friends. Removing this item from the scale would increase internal reliability. In the analysis stratified by gender this observation per- sisted, however, stratifying by case-control status (diabe- tes versus non-diabetes) this was true only for diabetic patients. For the purpose of standardisation we used the instrument as given and did not remove the sexual life sat- isfaction item from the social relationships domain. Characteristics of the sample Socio-economic information for the two groups is pre- sented in Table 1. Patients with diabetes were more often widowed, had lower levels of education and income, but were more often employed than non-diabetic controls. They were mostly patients with type 2 diabetes (92.4%) with a relatively long history of disease (72.6% over 6 years, 33.5% over 10 years). Despite the high proportion of patients with complications (80.2% with one or more self-reported major complication: nephropathy, neuropa- thy, retinopathy or diabetic foot), more than half were treated by oral medication (53.8%). Female patients had a longer duration of the disease and more self-reported complications than male patients (Table 2). Evaluation of the HRQOL The HRQOL was significantly lower for diabetic patients than for controls in all of the domains with the largest dif- ferences in the physical health and psychological domains (39 and 35 points difference) and smaller differences in social relationships and environment domains (19 and 13 points difference) (Table 3). Both groups had particu- larly low scores in the environmental domain with the smallest difference between groups, indicating the bad environmental conditions affecting HRQOL of diabetic patients and controls in a similar way. In the multivariate analysis (joint analysis for all domains) several interactions including group status (dia- betic patients vs. controls) and age were significant (p < 0.05) (Table 4). Female diabetic patients had consistently lower HRQOL for all domains than male diabetic patients, whereas the gender difference in controls was negligible. For example healthy women reported a 0.5 point better score in the physical health domain, but women with diabetes a 9.1 (-45.2-(-36.1) = 9.1) point lower score. Similarly, diabetic patients younger than 50 years had a considerably better HRQOL than older patients, whereas the age effect in controls was very small. Diabetic patients with a lower number of persons finan- cially depending on them had a decreased HRQOL, but non-diabetic patients with a lower number of persons financially depending on them had a slightly increased HRQOL. Lower income (below 200 $) had stronger effect Page 3 of 7 (page number not for citation purposes)