What is the difference between the median and the mean values for this set of data: 11, 9, 21, 6, 18, 2, 17, 8, 7.

Median: Reorder the data in numerical order, then select the value that lies directly in the middle of the data line - if there is an odd number of data, the median value is the middle number and if there is an even number of data, the median is the mean of the two middle points. 

i.e. 2, 6, 7, 8, 9, 11, 17, 18, 21. The median is 9. 

Mean: Find the total sum of the data and then divide this sum by the number of data points there are. 

i.e. 11+9+21+6+18+2+17+8+7 = 99

99 / 9 = 11. The mean is 11.

Therefore, the difference between the median and the mean is 11-9 = 2.