Answer: The required median of the given data set is 16.5.
Step-by-step explanation: We are given to find the median of the following set of values :
11, 13, 14, 15, 18, 20, 23, 26.
We know that
(i) If the data contain an odd number of values, then the mid-value is the required median.
(ii) If the data contain an even number number of values, then the average of the two mid-values will be the required median.
Since the given data contains 8 values, which is an even number, so we will consider two mid-values and those are 15 and 18.
The average of 15 and 18 is given by

Thus, the required median of the given data set is 16.5.