This project started in 1988 with a first wave of data collection (Test occasion 1, T1) for 1000 participants, randomly selected in the population registry of Umeå (Sample 1, S1). The participants were in the ages of 35, 40, 45, ..., 80 years of age when tested at this occasion. There were 100 participants in each age cohort, with a sex distribution that reflects that of the population, i.e. about the same number of men and women in the younger cohorts and about twice as many women as men in the older cohorts.
At T2, starting 5 years later, 1993, 86% of the participants of S1 returned for testing. Among the remaining participants, 9% had died during the five-year interval, 2% had moved from Umeå and 3% refused to be tested again. Another two samples, S2 and S3, were randomly selected and invited to participate at T2. The participants of S2 were as old as the S1 participants were at T1, i.e. 35, 40, 45,..., 80 years of age, whereas participants in S3 were as old as S1 participants were at T2, i.e. 40, 45, 50,...85 years of age.
When S1, S2, and S3 participants returned five years later, 1998, and when S1 and S3 returned still another 5 years later (2003), the return rates were surprisingly similar, i.e. about 85%.
At test wave 5 (T5), the amount of returning participants had dropped further down to 80%. This was because the participants had become 20 years older than they were when Betula started in 1988. Many of the participants were deceased and many were not able to return to the health and cognitive testing due to health issues. A new sample of participants, S6 were added to the study and they were between 25-80 years old.
In 2013, at test wave 6 (T6), there were in total 576 persons tested from S1, S3 and S6.
At every test occasion, each participant undergoes a careful health control including blood sample testing by a nurse and an extensive examination of memory by an experienced experimenter. At several occations some participants has also done brain imaging.