ASSESSMENT OF BIAS IN THE SENECA STUDY

Objective: To assess manifest bias in ageing effects, i.e. longitudina l changes due to unintended time effects or to selection. Design: Mixe d-longitudinal study in birth cohorts 1913-1918, with baseline measure ments taken in 1988/1989 and repeated in 1993, including a short quest ionnaire in non-responders. Setting: Full baseline and follow-up data were collected in nine towns in eight European countries including Bel gium, Denmark, France, Italy, The Netherlands, Portugal, Spain and Swi tzerland. Incomplete data were available from towns in Portugal, Polan d, Northern Ireland and Connecticut, USA. Subjects: Using standardized methodologies data were collected from a random age-stratified sample of elderly men and women, including a total of 1221 re-invited subjec ts from nine towns and 210 newly-invited subjects from three towns in 1993. Results: An overall retrieval of 50-74% of the former participan ts could be reached in towns that had previously participated (apart f rom one exception of 41%), where estimates of mortality varied from 10 % to 18%. There was a tendency for healthy and active persons to have a higher participation rate than others, as was the case for high educ ated newly-invited subjects compared to lower educational classes. For most of the variables used in the analysis of period effects, no evid ence of any undesirable period effect was found. In those instances th at period effects showed up to be statistically significant, coincidin g implausible cohort effects gave the impression that these were due t o instability of the estimation procedure. Conclusions: Non-participan ts may be less healthy and active than the participants. Only very lim ited unconvincing evidence to suggest unintended time effects was obse rved. This confirms the high standards of the methodology and of measu rements.