Feasibility of cohort studies in Estonia

OBJECTIVES: To describe the methodology and feasibility of follow up for vital status in retrospective cohort studies in Estonia. METHODS: A cohort of 7412 workers who had been employed at two factories in Tallinn between 1946 and 1988 was followed up for vital status from the date of first employment until death, emigration, or the end of the study, 31 December 1995, whichever occurred first. The cohort was electronically linked with the National Population Registry of Estonia that was created in 1992 and includes personal identification numbers of Estonian citizens and residents, and the Mortality Database that contains information from death certificates issued in 1983-95. A manual search was carried out on several non-computerised population data sources and archives. RESULTS: By 31 December 1995, the vital status of 6780 (91.5%) subjects could be traced (4495 (60.6%) subjects were alive, 1993 (26.9%) had died, and 292 (3.9%) had emigrated). Analysis by calendar period of leaving work showed that the proportion of subjects traced was lowest in the group of workers who had left work between 1946 and 1955 (58.4%), especially those whose age at leaving work was < 30 (53.2%) or > 60 years (42.3%). Among subjects who left work in 1956-65, 1966-75, and 1976-88, the follow up rate was 84.7%, 94.6%, and 98.2%, respectively. CONCLUSIONS: The findings, which are especially important for occupational epidemiology, confirm the feasibility of conducting retrospective cohort studies in Estonia. Most of the issues discussed in the paper apply to other former Soviet countries.