Math basics › Probability and statistics
Mean and standard deviation
The middle
Mean: add them up and divide by how many. Median: the middle value once they are sorted. An outlier drags the mean but barely moves the median.
The spread
Standard deviation is roughly the typical distance from the mean. Variance is its square.
Sample or population
For a sample, divide by n − 1 instead of n when finding the variance. Check which one your class wants.