Hands-On Data Analysis with Pandas

Standard deviation

The variance gives us a statistic with squared units. This means that if we started with data on gross domestic product (GDP) in dollars ($), then our variance would be in dollars squared ($²). This isn't really useful when we're trying to see how this describes the data; we can use the magnitude (size) itself to see how spread out something is (large values = large spread), but beyond that, we need a measure of spread with units that are the same as our data.

For this purpose, we use the standard deviation, which is simply the square root of the variance. By performing this operation, we get a statistic in units that we can make sense of again ($ for our GDP example):

The population standard deviation is represented as ...