There were potentially six questionnaires a participant could have completed around the time he/she entered the trial: the Baseline Questionnaire, the Supplemental Questionnaire, the Dietary Questionnaire, the Diet History Questionnaire, the Medication Use Questionnaire, and the Brief Survey Questionnaire.
A Supplemental Questionnaire (SQX) was introduced in 2006, 13 years into the trial. The contents of this questionnaire largely overlap with information collected on the baseline questionnaire (BQ). Some of the overlapping information is data that would not be expected to change between the BQ and the SQX (e.g. race-ethnicity). Other variables may reasonably change between the two questionnaires (e.g. marital status, smoking status, diseases). The SQX additionally contains some data not collected on the BQ (e.g., physical activity).
Ultimately, about 104,000 questionnaires were collected. Given the late administration of the SQX, the median time in the trial following a completion of this form is 3 years.
SQX data can be obtained in the SQX dataset