Statistics problems are not so bad on the GMAT.
You need to really understand what the mean is thoroughly, but if you remember that the MEAN = SUM divided by #ITEMS you cannot go wrong.
Don't forget, all you need to know to calculate a mean is the sum of the itmes and the number of items (you do NOT need to know what all of the items are).
Remember the mean is an average, the median is the "middle" number (or average of two middle numbers If the number of items is even), and "mode" (which I have never seen used in real life) is the item that occurs the most (the most might not be unique).
Finally, the only thing you have to know about standard deviation is that it is a measure of dispersity (how different and/or spread out the range numbers are).
Former Senior Instructor, Manhattan GMAT and VeritasPrep
Vice President, Midtown NYC Investment Bank, Structured Finance IT
MFE, Haas School of Business, UC Berkeley, Class of 2005
MBA, Anderson School of Management, UCLA, Class of 1993