Assessment of representativity of a study population - experience of the Kiel Obesity Prevention Study (KOPS).

OBJECTIVE Exemplified by data of the Kiel Obesity Prevention Study (KOPS), different methods to control for response bias and to assess representativity were compared. METHODS 4,997 cross-sectional data of 5- to 7-year-old German children (main cohort) were investigated between 1996 and 2001 within school entry examination. A subgroup responded to a questionnaire to socio-demographic and lifestyle factors (responders, n = 2,631). Representativity of the main cohort was tested in comparison to the total population. To control for response bias within the responders a non-response analysis as well as an analysis of missing values were performed. RESULTS The comparison with the total population showed a higher prevalence of obese boys and girls from families of low socio-economic status (SES) within the main cohort. The responders were less frequently obese and overweight and more rarely belonged to low SES families when compared with non-responders. Analysis of missing values did not detect any further biases. According to an epidemiological assessment of differences the main cohort of KOPS is suggested to be representative for all 5- to 7-year-old children in Kiel, whereas the responders can be at best called 'relatively' representative. CONCLUSION The analysis of non-response is the most sensitive method to detect group differences, but a comparison with the total population can also be used to control for biases. In addition representativity has to be proven not only for the main cohort but also for the subgroup of responders with which data analysis will be done.