Some obstacles to the evaluation and effective performance of health appraisals

Certain obstacles exist in evaluating screening tests and health examinations. When a voluntarily screened group is compared with an unscreened group and when the survival of cases detected in these two groups is compared, self-selection bias, lead-time bias, length bias and over diagnosis confuse the evaluation and usually make an examination seem better than it really is. A randomized controlled trial overcomes these biases; one group is offered screening, the other is not and the outcomes in both entire groups are observed. Unavoidable crossovers between the groups tend to obscure any benefit of the examination, however. Evaluations of screening tests have tended to emphasize the value of positive results and have neglected the value of negative results. In obtaining maximum benefit from health appraisals, attention needs to be directed at self-selection, nonmedical threats to life and health, risk factors in addition to early disease and tailoring an examination to the needs of individual patients.

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