Hi guys !

I have a quick question :

We don't understand why it is better to consider contrasts (linear combination of parameters : , where ) in the Scheffé method for simultaneous confidence intervals...

Why do we need the latter sum to be 0 ? I read in a lecture note that if we consider contrasts instead of any linear combination, we're considering a Fisher distribution with a degree of freedom that is J-1 instead of J. But is it really the only reason ?

Thanks for any input, even if it doesn't exactly answer the question... if it casts a light on some point of the Scheffé method, I would also be pleased