Calculating Probability for Mean Time Between Events

[spock0149]

Ok,

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

Question :

If the mean time between some random event occurring is 6 months, what is the probability 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

 

[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.

 

[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.

 

[spock0149]

Thanks for the leeds guys.

Spock

 

[spock0149]

how does this look:

rate =

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