The tendency for collected data to differ from what is expected in a systematic way. Biased data can often favor a specific group of those studied.
People have the tendency to change their behavior when they know they are being monitored.
Bias that occurs when a sample consists entirely of volunteers. People with strong opinions may be the only ones who volunteer.
The desire of researchers (and research publications) to only print the most sensational or interesting articles.
Bias that occurs when a respondent tells the interviewer "what they want to hear" or lies due to the sensitive nature of the question.
Selecting a sample in such a way that certain subsets of the population are systematically excluded.