Caption: Degrees of Freedom
Text: The number of values in a study that are free to vary. For example, if you have to take ten different courses to graduate, and only ten different courses are offered, then you have nine degrees of freedom. After the first nine course selections, there will only be one class left to take.

When estimating the population standard deviation from a sample, there will be n-1 degrees of freedom, because the deviations from the mean must add up to 0.