When a participant in one arm of a randomized trial also utilizes the intervention assigned to the other arm, it is not possible to attribute any benefit (or harm) experienced by the participant to the intervention that he or she was assigned. A high level of such "contamination" interferes with reaching proper conclusions from the trial's data. To assess whether contamination happened to a significant extent in NLST, an annual survey was conducted among LSS participants.
Each April from 2004 through 2010, a sample of 400-500 LSS participants beyond their baseline year whose previous screening result was negative with no significant abnormalities was selected at random to receive a Health Assessment Questionnaire (HAQ). The HAQ asked about eight types of medical procedures done over the previous 12 months. The primary purpose was to assess whether participants in each screening arm were also receiving the other screening modality outside the trial during the T0-T2 study years. Information about continuation of screening after the trial's three annual screens was also obtained through the HAQ.
The LSS HAQ dataset includes one record per questionnaire completed. Note that some participants filled out multiple forms in different survey years.