The quartiles are calculated as the 25th, 50th, and 75th percentile values in the data set. The quartile values may be represented as actual data values or may be calculated from actual data values.
Quartile 1 is the 25th percentile value, which is the value below which 25% of the values in the data set occur.
Quartile 2 is the 50th percentile value, but is always referred to as the median value. 50% of all values in the data set appear above this value, and 50% appear below this value.
Quartile 3 is the 75th percentile value, which is the value below which 75% of the values in the data set occur.
The quartiles of the values [5,10,15,20] are:
- Quartile 1: 7.5 = (5 + 10) / 2
- Quartile 2: 12.5 = (10 + 15) / 2
- Quartile 3: 17.5 = (15 + 20) / 2