A quartile is a commonly used term in the world of Statistics. The third quartile is the median of the second half of data. For instance, if there was 15 numbers, you would arrange them from least to greatest, and the second half of the numbers would be the last 7. Then the median of those 7 would be the third quartile, or Q3. Below is an example.
1  2  3  4  5  6  7  8  9  10  11  12  13  14  15

The median of the above data would be '8'. And in this case, the second half is already in order from least to greatest. Therefore, Q3 is 12.

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