Surveys were mailed to every 25^thperson listed in a local telephone directory. The purpose of the study was to examine the relationship between smoking and skin cancer. Each person was asked to report age, sex, smoking habits, and skin cancer presence during the preceding week. Nearly 30% of the surveys were completed and returned. About 10% of participants reported the presence of skin cancer overall. 35% of current smokers, 10% of former smokers, and 5% of never smokers reported skin cancer.

What is the likely impact of the 30% participation rate on the study's results or findings?