The Canon by Natalie Angier Page B

honors, and has been active in the nuclear freeze and antidiscrimination movements.
    Following that tapas of a biography are eight statements, which the readers are asked to rank in order of probability that they apply to Linda. Among them: Linda is a bank teller; Linda is a feminist; Linda is married and has two children; Linda lives in a university town; Linda is a feminist and a bank teller.
    Time and again, Koehler said, readers think they know Linda. She's a feminist—that they rank high. And she probably lives in a university
town. The married-with-kids part, who can say, so that gets a listing somewhere in the middle. But a bank teller? That description doesn't sound like Linda at all, and it earns an average ranking way at the bottom of the stack. She could, however, be a feminist
and
a bank teller, couldn't she? Readers assign the composite declaration a higher probability than bank teller alone. "Almost ninety percent of people do this," said Koehler. "They argue, she's definitely not a bank teller, but she could easily be a bank teller and a feminist. At least that's got some of Linda in there. That seems to be the way people think about probability."
    There is, of course, a higher probability of Linda being a bank teller than a bank teller and a feminist. In order to be a bank teller and a feminist, she must be a bank teller; and the unconditional probability of one event occurring—in this case, bank tellerdom—is always going to be greater than the conditional conjunction of that event plus a second event—bank tellerdom and a familiarity with the works of Simone de Beauvoir and Gerda Lerner.
    Yet even as people accept that Linda might be a feminist bank teller, they feel uncomfortable thinking of Linda's overall prospect of being a bank teller, period. Some might think that to use the job description alone negates, misrepresents, or shortchanges an essential aspect of her being, just as I've felt compelled to qualify my answer whenever people have asked what my father did for a living: he was a machinist at Otis Elevator Company, I say, but he was also an artist who made intricate pen-and-ink drawings, i.e., he was no Archie Bunker. Alternatively, people might be unconsciously fleshing out the statement "Linda is a bank teller" with a clause, "but she is not a feminist," to place it in direct contrast to the statement "Linda is a feminist and a bank teller."
    However understandable and folksy maybe the urge to rank the conditional above the unconditional premise in Likely Lines about Linda, it is incorrect, and when Koehler's students realize the error of their weighs, they feel foolish at first, and then eager to try the trick on family and friends, and finally liberated. Where else can they apply their newfound wisdom, their awareness of how important it is to consider background?
    Nowhere is the utility of sample-space tracing more obvious than when interpreting the results of a medical test. As a number of studies have revealed, doctors are not always skilled at estimating probabilities or putting a test result in proper context, which means that patients may be sent into paroxysms of anxiety, soul-searching, and planning of funeral choreography unnecessarily, or at least prematurely.
    Let's take as an illustrative but purely hypothetical example the following scenario. You're at the doctor's office for routine maintenance, and you happen to notice a sign advertising the monthly special: an AIDS test that is described as "95 percent accurate." You are not in any of the standard high-risk groups for the disease—though you did have crab lice back in college—but as a conscientious citizen and aspiring hypochondriac, you decide to roll up your sleeve and get screened.
    A week later, the receptionist from the temp agency who's been filling in for your doctor's phlebotomist calls with grim news: you tested positive. You feel the blood abandon your head and reconvene around your plantar