I guess all of you must be familiar with the formulae of Mean, Median and Standard Deviation {S.D} ... even if you are not its fine!! Good news is that GMAT does not test your knowledge of formulae involved..Statistics questions do pop up in the exam and following tips might come handy:

1. Standard Deviation gives the dispersion or spread of the data around its mean
* simply means that if standard deviation is large data is spread over a greater range around mean...or a lot of the data points are not close to the mean value

2. Adding or Subtracting a constant value from each term in the set has NO EFFECT ON SD

3. If each term in the set is multiplied by a number > 1 then SD increases

4. If each term in the set is multiplied by a number < 1 then SD decreases

5. If each term divided by a number > 1 then SD decreases

6. If each term divided by a number > 0 but number < 1 then SD increases .
let us say we divide each term by number 'n' , 0 < n < 1 then SD increases

7. Changing the sign of all terms in set has no bearing on SD..

8. If you are given multiple sets of numbers and you have to find out the set with largest and smallest SD just look at the set where the difference between the terms is greatest and the set where the difference between the terms is smallest respectively..

{ i hope the wording is not confusing, i know it sucks brevity per se }