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.