Afollow-up study was conducted on 3000 military troops deployed at an atomic test site in nevada to detect the occurrence of leukemia. a total 1870 people were successfully traced by investigators, and 443 contacted the investigators on their own as a result of publicity about the study. 4 cases of leukemia occurred among the 1870 people traced, and an additional 4 occurred among those individuals who contacted investigators on their own. could this be subject to bias?