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#### Alexmahone

##### Active member

- Jan 26, 2012

- 268

Would someone please explain this to me? It goes against my intuition as well.I manage a drug-testing program for an organization with 400 employees. Every three months, a random-number generator selects 100 names for testing. Afterward, these names go back into the selection pool. Obviously, the probability of an employee being chosen in one quarter is 25 percent. But what’s the likelihood of being chosen over the course of a year?

—Jerry Haskins, Vicksburg, Miss.

The probability remains 25 percent, despite the repeated testing. One might think that as the number of tests grows, the likelihood of being chosen increases, but as long as the size of the pool remains the same, so does the probability. Goes against your intuition, doesn’t it?

— Marilyn vos Savant

---------- Post added at 11:19 PM ---------- Previous post was at 11:05 PM ----------

I think the answer should be 1-P(chosen no times)$=1-0.75^4\approx 0.68$