Box Plot

Also called "box and whisker plots". Box Plots are used as graphical summaries depicting distributions. A histogram is more informative. By viewing the plots from the side it is possible to visualize the histogram’s general shape.

A sample result is shown below. The box plot shapes and spreads can provide quick general information on variation within and between categories. It may also indicate special cause or obvious areas of opportunity (outliers).

2)The first quartile (Q1) represents the point where 25% of the data are below it. It is represented by the line at the bottom of the box.

3)The third quartile (Q3) represents the point where 75% of the data are below it. It is represented by the line at the top of the box.

4)The whisker extend up to the highest value of upper limit and down to the lowest value of the lower limit.

5)The lowest point of the lower whisker is called the lower limit. Lower limit value equals Q1 – 1.5 * (Q3-Q1).

6)The highest point of the upper whisker is the called the upper limit. The upper limit value equals Q3 + 1.5 * (Q3-Q1).

7)Outliers are points that fall outside the limits of the whiskers. These are normally recognized as asterisks and are points that are greater than 1.5x the interquartile distance from each quartile. Each point that qualifies will have an asterisk.

8)The interquartile is represented by the distance between Q1 and Q3.

9)The widths of the box plot indicate the size of the sample taken. The wider the box, the larger the sample. This is usually an option to select in statistical software programs, not all box-plots have the widths proportional to sample size.

Box Plot of a Normal Distribution