No. You can't have "is not the same as" or "different" as your null hypothesis. It sounds like what you really want to do is "equivalence testing". This looks to be a decent explanation of why you don't want to do "standard" significance testing and a brief explanation of equivalence testing:

http://www.graphpad.com/library/biostatsspecial/article_182.htm
if i cant have "is not the same as" or "different" as my null hypothesis,

**so what should my null hypothesis be like?**
in my minor thesis, I compared the score of episodic memory between deaf and normal children.

my data distribution is not normal, so i used mann-whitney to compare, and it means I used median instead of mean.

there was no significant difference between the 2 groups (median score of deaf group=8; median score of normal group=9;p=0,256)

but from what i've read in your link, if i accept my hypothesis (deaf group score is the same as normal group score) based on that significance test, my conclusion is invalid.

it said i your link there's way to ask about equivalence and express the results as p value, but it's complicated. or it said that I could use CI.

but I didnt see any confidence interval in mann-whitney test.

so do u know,

**what test should I use?** and u should know beforehand that I'm no expert and I cant understand a really difficult statistical things.

thank u so much for ur help