1. ## sample size

i have a formula for the sample size as
n=A * [ ( (a+t)(1-a-t) + a(1-a) ) / t^2 ], where A, a & t are all unknown constants.
The question is to show that n has a maximum when
a = (1-t)/2

I think the A can just be ignored so that I need to find the maximum of the top line and minimum of the bottom. Since the bottom is just t^2 and fixed I can't do anything with it so just need to maximise the top?
At first I differentiated the top with respect to t and ended up with a = (1-2t)/2, although since t is constant I don't think I can do this?

Any help would be very welcome!

2. ## Re: sample size

Originally Posted by bubblygirl90
i have a formula for the sample size as
n=A * [ ( (a+t)(1-a-t) + a(1-a) ) / t^2 ], where A, a & t are all unknown constants.
The question is to show that n has a maximum when
a = (1-t)/2

I think the A can just be ignored so that I need to find the maximum of the top line and minimum of the bottom. Since the bottom is just t^2 and fixed I can't do anything with it so just need to maximise the top?
At first I differentiated the top with respect to t and ended up with a = (1-2t)/2, although since t is constant I don't think I can do this?

Any help would be very welcome!
You need to differentiate the top with respect to a.

3. ## Re: sample size

Originally Posted by alexmahone
You need to differentiate the top with respect to a.
Ahh, so simple! Don't know why i didn't see that

Thank you!