Non-response bias
Non-response bias is the potential skewing of results due to people not responding. When people who choose not to respond to a survey are systematically different from those that DO choose to respond, non-response bias occurs. The result of non-response bias can be that a sample does not accurately represent the population of interest.
Evaluators can assess non-response bias if they can convince the people who didn’t respond to take a shorter survey.
See also: bias