when should i use this? my book just kinda throws it in there in a proof

some of the solutions show it using SP and others dont

is it only when im trying to find Ux-Uy?

/e statistics stuff
SP = sqrt( (n-1)sx^2 + (m-1)sy^2 / n+m-2)

where Sx,Sy are estimates of variance and n, m are sample sizes

This post was edited by brigadier on May 1 2013 11:27pm

Use it when you have two groups with different number of individuals. Eg. in a population, there are 23 males and 25 females.
Basically you're calculating a pooled variance (which is then the pooled sd).

This post was edited by Exx on May 2 2013 09:20pm

makes sense