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USING STATISTICS: Spotting and Avoiding Them
Misunderstandings arising from
lack of clarity about the reference
Gigerenzer et al1
physician may tell a patient
that if he takes a certain antidepressant, his chance of developing a
sexual problem is 30% to 50%. The patient may interpret that as saying
that in 30% to 50% of the occasions
on which he wishes to have sex,
he will have a problem. But the
physician means that 30 to 50% of
his patients who take the medication
develop a sexual problem.
The physician has intended
one reference category (patients who
take the medication) for the percent, but the patient has heard another
(occasions on which he wishes to have sex). The physician's phrasing
has in fact "led" the patient to this interpretation, by stating the
risk in terms of the individual patient ("his chance"), rather than in
terms of the
category of all patients who take the medication.
1. Gerd Gigerenzer, Wolfgang Gaissmaier, Elke Kurz-Milcke,
Schwartz, Steven Woloshin (2007), "Helping Doctors and Patients Make
Sense of Health Statistics,"
Science in the Public Interest,
vo. 8, No. 2, pp. 53 - 96.
Download from http://www.psychologicalscience.org/journals/index.cfm?journal=pspi&content=pspi/8_2