The most important aspect of evaluating a  screening program is whether it is effective in reducing morbidity, disability and mortality from the disease. A number of factors can be evaluated within a short period  of time after initiation of a screening program as severity of disease at the time of diagnosis. However, such measures are difficult to evaluate as participants in screening may be more healthy conscious and thus more likely to have their condition discovered at early stage than non-participants regardless of the screening itself. The most definitive measure is a comparison of the cause-specific mortality rates among those discovered by screening and those diagnosed by development of symptoms. Both groups must be comparable with regard to all factors affecting the endpoint under evaluation with the exception of the screening experience. As well as they should be equally scrutinized for ascertainment of outcome.