# Basic stats questions

1. Sep 10, 2006

### spock0149

Ok,

This problem sounds really easy, but I think I am doing something wrong.

Question :

If the mean time between some random event occuring is 6 months, what is the probablity that in one year the event does not happen.

I think its like flipping a coin. There is a 0.5 chance of the event NOT happening in 6 months, so there is 0.5 x 0.5 chance of it not happening in 1 year, so P = 0.25.

Does this sound right?

Thanks

2. Sep 11, 2006

### robert Ihnot

That could not be in generarl correct: take the case of a baby averaging nine months between conception and delivery. Time difference would be pretty much restricted by this 9 month figure.

And even if someone was to argue that what in medicine is "normal," is not the same as the "mean," well take the "normal temperature" of 98.6F; if you took the temperature of 10,000 people, I am sure the mean result could not vary much from that, probably by less than 2 degrees F.

Last edited: Sep 11, 2006
3. Sep 11, 2006

### daveb

What you have a classic case of an exponential distribution with 6 months as the rate parameter. This should be enough for you to figure the problem out.

4. Sep 11, 2006

### spock0149

Thanks for the leeds guys.

Spock

5. Sep 16, 2006

### spock0149

how does this look:

rate =

$$e^{-\frac{\labmda}{t}}=e^{-\frac{12}{6}}$$
so,
probability of event occuring in one year = 1-rate =0.865

Last edited: Sep 16, 2006