#### ElbowPatches

##### New member

- Joined
- Apr 30, 2017

- Messages
- 3

I'd like to know how to calculate an expected value for the number of a population with a particular property.

Let's suppose that for the whole population of a country 20% of those who choose to study a subject, let's say mathematics at A level, are female. (Let's say the population has a 1:1 ratio of male:female as a whole, though I'd also like to account for there being a 51:49 ratio or such nationally).

If I take a year group that has 50 males and 50 females, I'd expect the 4:1 ratio of male:female choosing to study maths, regardless of the total number that choose it.

What if my group doesn't have a 1:1 ratio, but instead has say 40% males and 60% females. What proportion of males and females would I expect to choose maths?

Would it be 24%, as an additional fifth of female population are there with the same expected chance of choosing maths?

What would be the general formula for working out the expected proportion of m:f in a subject, given an initial proportion of m:f in the yearvgroup?

Thanks for any help everyone.

EP