Sample or population?
Use the sample standard deviation when your numbers are a sample drawn from a bigger group, which is the usual case in class and in research. Use the population figure when you have every value in the group. The sample version divides by n − 1 instead of n.
How the median and quartiles are found
The numbers are sorted, and the median is the middle value, or the average of the two middle values. Quartiles use linear interpolation between the closest ranks, the same method as Excel's QUARTILE.INC and most graphing calculators.
Mode
The mode is the value that appears most often. A list can have more than one mode, and if every value appears only once there is no mode.