The median is an estimate of central tendency, and is calculated as the middle of the values, sorted numerically.
This is the actual middle value when there are an odd number of values.
However, when there are an even number of values, the median is the average of the two middle values.
The median of the values [5,10,20] is 10
The median of the values [5,10,15,20] is 12.5 = (10 + 15) / 2