Science warns me to be careful how I adopt a view which jumps with my preconceptions and to require stronger evidence for such belief than for one to which I was previously hostile. My business is to teach my aspirations to conform with fact, not to try and make facts harmonize with my aspirations: Thomas Huxley, 1860 A habit of basing convictions upon evidence, and of giving to them only that degree of certainty which evidence warrants would, if it became general, cure most of the ills from which the world is suffering: Bertrand Russell, 1957 Especially in the emotionally charged field of cancer screening, which can have substantial public health implications for large numbers of healthy, asymptomatic people, it is important to achieve strong levels of evidence before promulgating new screening tools. This review of screening study methodology is intended to help the reader weigh such evidence and to evaluate reports which appear in the literature. It is an attempt to go beyond the often-stated intuition that early cancer detection finds cancers when they are easier to treat, at a time when survival is best. Examples tell us that sometimes this assumption has been true, sometimes not. A familiarity with the hidden biases in the supposition can be translated into everyday medical practice for screening tests in general. The practitioner can then match the strength of recommendation with the strength of the evidence behind the recommendation.
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