From October 2011 to June 2012, we approached all available twins in the same-sexed twin cohort irrespective of their zygosity or earlier response status. Overall we sent the questionnaire, either in Finnish or Swedish (Finland being a bilingual country) to 11,738 twins born 1945–1957, originally identified in 1974 when the cohort was established. Their vital status was updated in 2011 from the Finnish national population register and all twins known to be alive with an address in Finland formed the target population for this survey. We obtained 8,501 responses, a response rate of 72%. Data entry and cleaning are in progress, so the numbers may still change somewhat. The questionnaire was mailed, rather than administered by internet, given the age of the subjects and the fact that the three prior questionnaires had also been paper ones. Therefore, a change of mode of administration was not warranted in order to retain comparability across waves.
