Using Quartiles

Quality calculates the twenty-fifth, fiftieth, (also referred to as the median), and seventy-fifth quartiles. The quartiles can be displayed as values and are used to graph the Box and Whisker plots.

To compute the quartiles, the system:

1. Arranges data in ascending order.

2. Ranks the data accordingly (1 to n).

3. Multiplies each quartile by n+1.

4. If the result is an integer, sets the quartile to the value of the calculated rank.

The following table shows quartile equations:

Equation	Statistic
	The median is the center or middle of a sample. It is the value above which there are as many values as there are below it. It is also the fiftieth percentile of the sample (Quartile 50 percent). See References.
	The twenty-fifth percent quartile is the point separating the lower 25 percent of the values from the upper 75 percent. See References.
	The seventy-fifth percent quartile is the point separating the upper 25 percent of the values from the lower 75 percent.
where: p is the percentile, f is the fractional portion of the computed rank, I is the integer portion of the computed rank.	To resolve calculated values that are not integers (for example, if the percentage lies between two values), the value is interpolated by calculating the weighted average between the two ranks. See References.