Research done on pedophilia and other criminal acts by interviewing prisoners is subject to sampling bias.
Reasons for sampling bias:
(1) If the discussion is about a criminal act, then the subject may deny things rather than to give evidence to prosecutors.
(2) A person who had committed a less serious form of the crime might not be in prison.
(3) A person less impulsive might not be in prison.
(4) A person with money, power or influence may be able to avoid prison.
(5) Racial, gender and/or age bias in the legal system.
(6) Small sample size.
