A quartile is a commonly used term in the world of Statistics. The first quartile is the median of the first half of data. For instance, if there was 15 numbers, you would arrange them from least to greatest, and the first half of the numbers would be the first 7. Then the median of those 7 would be the first quartile, or Q1. 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 first half is already in order from least to greatest. Therefore, Q1 is 4.

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