Measures of Centre

The Mean
The mean of a set of observations is the average value of those observations. It is found by adding all the observations together and dividing the result by the number of observations. This is shown by the formula

The mean can be greatly influenced by outliers.

The Median
The median is the middle value in a distribution, half the observations fall below it, and half above.
e.g.
The median of
1,2,3,4,5
is 3.

If there are an even number of observations, the median is the mean of the two middle numbers.
e.g.
The median of
1,2,3,4,5,6
is the mean of 3 and 4, which is 3.5.

As the median is the value in the middle of the observations, it is relatively unaffected by outliers. If the final number in the data set above was 999 rather than 6, the median would still be 3.5. Because of this, the median is said to be a resistant measure of centre.