If the probability of observing a car (read: at least one car) in 20 minutes on a highway is 609/625, what is the probability of observing a car (read: at least one) in 5 minutes (assuming constant default probability)?
P(at least one car) = 1  P(no cars)
So the probability of no cars in 20 minutes = 1  609/625 = 16/625
There are four 5minute intervals in 20 minutes. Assuming they are independent, P(no cars in 20 min.) = P(no cars in 5 minutes)^4
P(no cars in 5 min) = 2/5
The probability of at least one car is then 3/5.
I think the independence assumption is probably not valid in the real world, but the problem seems to indicate we can.
Jer

Posted by Jer
on 20040409 12:13:22 