"In research studies that are retrospective, such as case-control studies, the researchers start with a series of cases (those who have the disease) and a series of controls (those who do not have the disease) and compare them. For example, an epidemiologist might want to investigate the potential effect of influenza infection, occurring during the early stages of pregnancy, and its potential role in the development of certain birth defects. The cases, would be mothers of children with the birth defects. A requirement in such research is an accurate memory on the part of both cases and controls. However, it is likely that mothers of children born with birth defects would have closely examined their pregnancy searching for clues or reasons that might explain the birth defects. If they had had the flu, they would no doubt remember it and might also remember it being more severe. On the other hand, mothers of normal babies are much less likely to remember having had the flu during pregnancy, especially if the pregnancy was several years back. This heightened likelihood of memory is called
recall bias."1"Recall bias is a poorly understood but well-recognised problem in retrospective studies and can substantially alter the frequency of symptom reports. Media reports and health care professionals may have sometimes unwittingly led some people e.g. Gulf War veterans to become overly concerned about the possibility of developing chemical sensitivity or other unexplained illnesses and to report more symptoms."2