Risk factors as prognotic tools

There is a very nice 'perspective' in December 21, 2006 issue of New England Journal of Medicine by James Ware. It discusses that a risk factor must have a much stronger association with the disease outcome than we ordinarily see in etiologic research if it is to provide a basis for early diagnosis or prediction in individual patients. It clearly explains that If the distributions of the risk factor differ between the group that will have the outcome and the group that will not, the risk factor is associated with the outcome. If the sample size is sufficiently large and the model is properly specified, one can expect to show that the risk factor is a statistically significant contributor to a prediction model for the outcome. For the risk factor to perform well as a prognostic test for the individual patient, however, the distributions in the two groups must be sufficiently well separated to permit the selection of a cutoff value that will discriminate between the two groups with high sensitivity and specificity.