# Thread: Not sure what distribution to use [Weilbull?]

1. ## Not sure what distribution to use [Weilbull?]

I'm not sure how I should go about doing this problem

The maintenance manager at a chemical factory knows the number of repairs per year follows a Poisson process, and that on average there are 60 failures per 10 years and these occur in a random fashion over the years. Expressing probability as a number between 0 and 1.

What's the probability that a randomly selected 30 month time period contains at least 2 repairs?

Can anyone give me some guidance please?

Thanks

2. ## Re: Not sure what distribution to use [Weilbull?]

It's Poisson with $\lambda=\dfrac{60}{10}=6 \text{ failures per year } = 0.5 \text{ failures per month }$

Since the failures follow a Poisson process any 30 months is statistically the same as any other 30 months across the 10 yrs there is data on.

In 30 months we expect 15 failures so we set $\lambda=15$ and evaluate the 1 - the CDF of the Poisson distribution evaluated at 1.

I get $p=0.999961$

3. ## Re: Not sure what distribution to use [Weilbull?]

Thank you very much ^^