Selection bias must be considered in assessing the results of any screening effort. The population most likely to seek screening may differ from the general population to which the screening test might be applied. The individuals screened may have volunteered because of a particular risk factor not found in the general population, such as a strong family history. In general, volunteers for studies are more health conscious and likely to have a better prognosis or lower mortality rate, irrespective of the screening result. This is termed the healthy volunteer effect