Statistical Error

Medical specialist Neil Schram, in his piece on AIDS statistics (Op-Ed Page, Aug. 24), misapprehends fundamental mathematical principles.

Taking it as axiomatic that a single act of unprotected vaginal intercourse with an infected partner carries with it a one in 500 chance of infecting the uninfected partner, Schram draws the erroneous conclusion that “500 acts of intercourse with one infected partner . . . leads to a 100% probability of infection (statistically, not necessarily in reality).”

Does Schram believe that every time a coin is tossed twice there is a 100% statistical chance that heads will come up?

Accepting Schram’s one in 500 probability estimate, the chance that such an event will happen at least once in 500 trials is actually about a 63% statistical probability.

DAVID G. STOUGHTON

Santa Monica