RAH, et al., It is true that one can combine two diagnostic tests to a worse effect than either alone, but it is not a foredrawn conclusion. To take a medical example, you screen first with a cheap test that has low/no false negatives then for the remaining positives you screen with a potentially more expensive test that has low/no false positives. There is a whole health policy & management literature on this. I reproduce the barest precis of same below, assuming the reader can manage to view it in a fixed width font while respecting my hard carriage returns as writ.
--dan cheat sheet on terminology of medical diagnostic testing _________________________________________________________________ \ the true situation \ \ + - +-------+-------+--- | | | + | a | b | a+b what the | | | diagnostic +-------+-------+--- test returns | | | - | c | d | c+d | | | +-------+-------+--- | | | | a+c | b+d | t true positives a = positive testers who have disease true negatives d = negative testers who are without disease false positives b = positive testers who are without disease false negatives c = negative testers who have disease prevalence (a+c)/t = fraction of population that has disease sensitivity a/(a+c) = what fraction of those with disease test positive specificity d/(b+d) = what fraction of those without disease test negative predictive value positive a/(a+b) = what fraction of positive tests have disease predictive value negative a/(a+b) = what fraction of negative tests are without disease Notes: Information retrieval people know sensitivity as "recall" and predictive value positive as "precision." Screening with a cheap test with high sensitivity then an expensive test with high specificity is often the best (most cost effective) strategy. --------------------------------------------------------------------- The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to [EMAIL PROTECTED]