@Carl Love  thanks, 

thats really helped :)

@Carl Love 

many thanks,

and does this means that the null hypothesis  is just  X > Z or X >= Z ?

because what I want is to be able to reject the two cases: X>Z and X=Z and accept X<Z in one go .

@Carl Love Hi, thanks for your reply and sorry, I think my explanition wasn't that clear :(

yes actually I want to do a t-test for paired sample means

I thought it would be something like the below, according to the link here

TwoSamplePairedTTest(X,Z,0,confidence=0.95,alternative='lowertail')

the only thing that I don't think I understand is does the lower tail means the following: 

- null hypothesis of : M1-M2 >= 0

- alternative hypothesis of M1-M2 < 0

if not how can get the results of the hypothesis that i want ?