1. ## Calculating Probability

I have not learned this in school yet, so have no idea how to do this, nor does my maths teacher. For an assignment i am talking about a mental disorder and thought it would be cool to show how many years it would take before you have a 25% and 50% chance of experiencing it.

This particular disorder effects a random 1/200 people each year. I need to know how to calculate this :S is there some particular formula someone would be able to show me?

Thanks very much for any help

2. Originally Posted by Ploppies
I have not learned this in school yet, so have no idea how to do this, nor does my maths teacher. For an assignment i am talking about a mental disorder and thought it would be cool to show how many years it would take before you have a 25% and 50% chance of experiencing it.

This particular disorder effects a random 1/200 people each year. I need to know how to calculate this :S is there some particular formula someone would be able to show me?

Thanks very much for any help
Is the probability of being effected in a particular year independent of being effected in other years? Or does the probability change over time (more likely as the number of years unaffected increase)?

3. every year the probability of any individual being effected is 1/200, so over the years that you are not effected the probability of you being effected increases

4. Originally Posted by Ploppies
every year the probability of any individual being effected is 1/200, so over the years that you are not effected the probability of you being effected increases
If the probability did not change, then the random variable would follow a geometric distribution with p = 1/200 and you need to calculate the values of k such that Pr(X = k) = 0.25 and Pr(X = k) = 0.5.

However, you've said that the probability changes over the years that you're not affected. In which case, some knowledge about the nature of this change is required .... The above will not hold true if p changes (but may be a reasonable approximation if the change is very small over the years).