According to The Post, a recent study published in The New England Journal of Medicine "questions the theory that Type A behavior leads to a higher risk of getting heart disease" {news story, Jan. 14}.

The new study does nothing of the sort -- it deals with conditional probabilities, not absolute ones. Restricted solely to individuals already identified as having heart disease, the study tells us simply this: once an individual has acquired heart disease, he's probably better off being a Type A.

This point, which is not all that implausible, can perhaps be better understood through exaggeration. Suppose the country were to be divided evenly between Type A and Type B personalities, with 100,000 Type As developing heart disease each year and only one Type B developing it. But suppose the Type B always died, while half of the Type As survived. If you knew you had heart disease, which would you rather be: a Type A or a Type B? A Type A, of course. That way, you would stand a 50 percent chance of surviving. But suppose you had not yet developed heart disease -- and wished to avoid doing so -- which would you rather be: a Type A or a Type B?

Only a fool or a medical researcher lacking analytical acumen would choose the former. (Note that more than two-thirds of the subjects in the reported study were Type As!) LEONARD GREENBERG Reston