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 differences by incentive group in a panel of 15 short tandem repeat (STR) loci used to identify genetic differences between samples for quality control of sample handling and processing [13]. These analyses suggest that in studies with low response rates, estimates may be influenced by factors such as family history, education or behavioral characteristics.
