a must read article in seomoz here it might be old for many of you here but that is not the only reason to post it here i need an explanation for the charts here
Since you NEED it, I'm sure it's life threatening. If that's the case, why didn't you ask specific questions instead? Look up "mean", look up "standard deviation", done.
Icheb let me tell you something,when you have to learn things with secondary language sometimes you need good guys to help you so please try to be good guy and instead of blaming me give me some useful informations here thank you .