The median of a data set is the number in the middle of a list when the data set is ordered from smallest to largest.
To find the median, list the numbers from least to greatest value:
![8,9,9,10,10,11,12,12,12,12,14,15,15,17,18](https://img.qammunity.org/2023/formulas/mathematics/college/fx7i8m67wn5g858gezhnfnidaw8d6awklz.png)
The list has 15 elements, then, the element in the middle of the list is the number:
![(15-1)/(2)=(14)/(2)=7\text{ th}](https://img.qammunity.org/2023/formulas/mathematics/college/zsztx26m5u8tazhv9qmwo7zbfkvpch5l5e.png)
The 7th element of the list is 12.
Therefore, the median of the data set is 12.