To further evaluate the impact of incentives on study participation, we explored the socio-demographic differences between control subjects who were enrolled with few or no incentives (~49% response rate) and those who were enrolled with the improved procedures (~73% response rate). We included in these analyses all controls recruited during the pilot studies and those recruited in the main study up to March 2003 (N = 748). We found some suggestive associations: the high incentive group exhibited an increased family history of lung cancer (p = 0.03); in addition, borderline associations were observed for: awareness of the link between smoking and lung cancer (lower, p = 0.07), anxiety score (lower, p = 0.08) and depression score (higher, p = 0.13) as measured by the Hospital Anxiety and Depression Scale (HADS), intention to quit smoking (lower, p = 0.10), history of quit attempts (lower, p = 0.11), military service (lower frequency, p = 0.11), and percentage attending college (lower, p = 0.12). Adjustment for age, gender, and, when appropriate, smoking, did not substantially alter these findings. We also found small non-significant
