Measures of Center

Briefly

A measure of center (such as the mean) is a statistic that gives you a value for an attribute that represents the entire population or a group.

Here are two common measures of center:

mean

The "average" value: add up all of the values and divide by the number of values. In Fathom, use the mean( ) function

median

The "middle" value: sort the values and choose the middle one. If there are an even number of values, take the mean of the middle two. In Fathom, use the median( ) function

Example: the numbers of tornadoes in the state during five successive weeks are {4, 20, 1, 3, 2}. The mean is 6 and the median is 3.

We call median a "resistant" measure because it is resistant to the effects of outliers. In the example, if the 20 were 100, the mean would be 21--but the median would not change.

Choose your measure according to what you need to do with it. To estimate the total number of tornadoes you would get during a four-week period a year from now, use the mean. On the other hand, the median is a more "typical" value. Here are some observations about choosing a measure of center: