Participants in a study may omit important information, or refuse to answer altogether because they are busy. These are called "nonresponse", and can influence the results of a survey.
ęDr. Quan, 2007
The human factor:
In carrying out epidemiological studies, the fact that the data is being collected from humans is a very important fact to remember. The data that is collected may sometimes not be completely accurate, because participants in a study might recall the information differently than what actually happened-for example, someone may accidentally omit a small, but important piece of information, or just not remember details correctly. This is always something that must be kept in mind when conducting a study. What do epidemiologists do to combat this potential discrepancy?