Follow-up and attrition. The follow-up period of the evaluations ranged widely from immediately after intervention to 2 years. Interestingly, one program that recruited participants both online and in clinics altered the timing of their follow-up according to recruitment mechanism . The Internet sample was asked to complete a follow-up risk assessment at 2 months, whereas the clinic sample completed their assessment at a 3-month interval more commonly used in health research. According to the authors,this approach was used to “avoid precipitous attrition at three months as seen in other research conducted with people recruited exclusively on the Internet.” Attrition rates in the 11 studies ranged from 3% to 57%, mostly depending on the length of follow-up. The lowest attrition rates were seen among college students [8]. High attrition rates were attributed to the difficulty of retaining samples in online studies and high rates of mobility and school dropout in studies that were conducted in low income areas . In none of the studies, however, was attrition reported to significantly bias the results.