Course Content

Learning Statistics with Python

The next term is **standard deviation**; this value is similar to the variance because **standard deviation** is a square root of the variance; hence the formulas will vary for the population and sample.

**Definition**

**Standard deviation** is a measure of how data spread out in relation to the mean.

**68–95–99.7 rule**

In statistics, one standard deviation or sigma plotted above and below the mean value corresponds to 68% of data; two sigmas include 95% and three 99.7.

**Example**
Let's figure it out with the sample of kittens' weights in grams.

Look at the picture; in this case, we deal with such values:

- Mean value = 100 grams.
- Standard deviation (sigma - the sign you can see on the picture) = 20 grams.

So, as we said, the one sigma above and below the mean corresponds to 68% of values. In this case, 68% are the values from **mean - standard deviation = 100 - 20 = 80** to **mean + standard deviation = 100 + 20 = 120**.

Section 3.

Chapter 4