**Re: if i use mann-whitney,is it okay if i still use mean (not median) to compare 2 gr**

Hi. I'm no statistical expert so please read all this with a pinch of salt and check with your supervisors...

I'm guessing the forward and backward digit span scores are number of digits remembered forwards and backwards of a predetermined number? I'm guessing, therefore, that that would be ordinary numeric data. The same for the word list memory task. In other words, remembering 4 digits is twice as good as 2 digits and 4 times as good as 1 digit. It would only be ordinal if the scores bear no quantitative relationship to each other.

You should probably use median if you are going to describe non-normal data, as the median should provide a better summary of a skewed dataset (mean and SD only describe a normally distributed dataset well, otherwise they are not so useful). And it's probably best to use non-parametric statistics such as Mann Whitney test to start with.

However, you may be able to use parametric tests if your sample size is large enough, due to the central limit theorem which states that the sample means of infinite samples taken from a population will converge to a normal distribution even if the underlying distribution is not normal. Since t-test, etc, is concerned with means and not the data itself, this can sometimes allow you to use parametric tests on non-normal data. The usual cut-off for this is 30 or so samples (you have n=15) but this cut off is arbitrary. So you could perform random sampling of your dataset to see if the means of the random samples converge to a normal distribution. I'm not sure, however, if this would be a valid thing to do, but it might work. On the other hand, the safe option is just to use the non-parametric stats.