Measures of Spread

Briefly

A measure of spread (such as the standard deviation) is a statistic that tells you about the variation in an attribute.

Here are two common measures of spread:

standard deviation

Use this seemingly odd procedure: take all the deviations of the values from the overall mean. Square them. Then add them up and divide by the number of values. (So you have the mean of the squares.) Then take the square root of the result. This is also called the root-mean-square deviation.
Here is the formula, where x
i are the individual values and is the overall mean

In Fathom, use the function popStdDev( ) or s( ). What's the difference?

interquartile range

In Fathom, use the function iqr( ). About calculating IQR.

Which one you use depends (as with measures of center) on what you need. Here are some considerations: