Hands-On Data Science and Python Machine Learning

Population variance versus sample variance

There is a little nuance to standard deviation and variance, and that's when you're talking about population versus sample variance. If you're working with a complete set of data, a complete set of observations, then you do exactly what I told you. You just take the average of all the squared variances from the mean and that's your variance.

However, if you're sampling your data, that is, if you're taking a subset of the data just to make computing easier, you have to do something a little bit different. Instead of dividing by the number of samples, you divide by the number of samples minus 1. Let's look at an example.

We'll use the sample data we were just studying for people standing in a line. ...