The median is generally, a better measure in valuation analysis as it is the CENTRAL value in a data group. The central value is found as follows:

MD = (n+1)/2

Summerton and Williamville have 19 sales each:

MD = (19+1)/2 = 10

Therefore, the median sale is that sale at the 10th place.

In Libre Office Calc (a free spreadsheet) : MEDIAN(cell range)