Saturday, May 8, 2010

ppv v fp

When asked about the accuracy of a mammogram, doctors cite the "false positive rate". Ignore the false positive rate, what patients really need to know is the "positive predictive value" (PPV), that is, the chance of having breast cancer given that one has a positive mammogram.

The PPV for mammography is 9 percent. Nine percent! You heard right. For every 100 patients who test positive, only 9 have breast cancer, the other 91 do not. This may sound like a travesty but it's easily explained: breast cancer is a rare disease, afflicting only 0.8% of women so almost all women who take mammograms do not have cancer, and even a highly accurate test when applied to large numbers of cancer-free women will generate a lot of false alarms.

Junk Charts on Steven Strogatz's piece for the NYT on Bayesian reasoning

1 comment:

Anonymous said...

It seems to me that Junk Charts picked wrongly a piece of Steven Strgatz's piece, when assuming "breast cancer is a rare disease, afflicting only 0.8% of women ", omitting that the satitistis were about these women who are in "a low-risk group: 40 to 50 years old, with no symptoms or family history of breast cancer", and not all women. Breast cancer is not a rare disease, it affects about 10% of women.