Box-Plot

See Also:

Quartiles Report

Levene's Test

Bartlett's Test

Box Plot Text

 

Description:

 

This graph is a box-whiskers plot for the parameter indicated in the selection bar. The graph is displayed for all sampling locations.

 

 

Use:

 

Box plots are a graphical method used to determine if sample distributions have equal variance. Levenetest and Bartletttest are analytical methods that also test equality of variance.

 

 

Implementation:

 

The box-whiskers plot is a method of visualizing the distribution of sample concentrations. The top-most and bottom-most points on the plot (the whiskers) indicate the maximum and minimum concentrations respectively. The "box" portion is made up of the 25th, 50th, and 75th percentiles of the concentration distribution.

 

 

Remarks:

 

The xth percentile of a distribution is the value below which x percent of the measurements lie. In many cases, the percentiles for a box plot do not produce an integer number. For example, in a distribution with 7 measurements, the 3.5th sample is the 50th percentile. The 1989 guidance recommends averaging the upper and lower values, in the case average the 3rd and 4th values in the distribution to get the 50th percentile.

 

ChemStat implements a weighted average between measurements. In testing, calculated values differed slightly than those reported in the 1992 guidance, but matched exactly with the values obtained with Microsoft Excel(TM). The difference between the two methods is typically so small, that it should not have any noticeable effect on the box plot.