F test

[chay dolly]

in mahlers sample exam # 4, i did it with x on top, leaving me with 9/24 * W. mahler flips it around. how do you know when to flip?

[NewTubaBoy]

The reason you know to flip is becuase with the F-test you always want to have the variance with the HIGHER variance on the top. So, in order to make the f-statistic, you know that the Y's should go on top and the X's shoudl go on the bottom. You know this since the alternative hypothesis has sigma2y > sigma 2x. So you create your F statistic, which is the reciprocal of the value of "W" that they give you. If you do the algebra, this will be why he did it that way.

If you do it your way and have X in the top, I think your degrees of freedom may turn out backwards... meaning you'll have F(24,9) instead of F(9,24).

[chay dolly]

you the man

[NewTubaBoy]

I only knew this because I spent about 20 min on that problem earlier today trying to figure out what the heck he did.