Cheap blood test detects pancreatic cancer before it spreads

Re:Cheap blood test detects pancreatic cancer before it spreads by dillydogg on Thursday February 13, 2025 @09:58PM (#43044290)

There was a seminar given to the breast cancer society by an epidemiologist years who who presented them with a scenario:

Prevalence of breast cancer: 1%

Sensitivity (percent of people who have the disease test positive): 90%

Specificity (percent of people without the disease test negative): 91%

And asked, "How many people who test positive have the disease?" (i.e., positive predictive value)

It's only 1/11. I think only 20% got it right (in a 4 answer multiple choice question)

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Re:Cheap blood test detects pancreatic cancer before it spreads by mlyle on Thursday February 13, 2025 @10:43PM (#43044600)

Yah, I'm well familiar with the base rate fallacy and I still catch myself screwing it up. It's so unintuitive. I use mental math a lot to counter my intuition.

If sensitivity is high and the base rate is low, you can approximate it with .01 / (1-.91).

Or, mental math assuming 10,000 people is not unreasonable for your case (100 with disease, 90 true positives; 9900 * 9/100 false positives.. divide everything by 9 to make it easier, 10 true positives and 99 false positives, or 10/109 or say "9%".

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