Preparing Boxplots in Excel A boxplot, or box and whisker plot, is a common way of displaying statistical data. It breaks the data up into 25% groupings, called quartiles. By looking at the five number summary, or the quartile boundaries, we can see which parts of the data are most spread out or dispersed.

Preparing Boxplots in Excel To prepare a boxplot we must first find the five number summary of the data. The five number summary consists of the quartile boundaries, Q0, Q1, Q2, Q3 and Q4. The value Q0 is just the smallest value, or minimum of the data. The value Q4 is the largest value, or maximum of the data. Q1 to Q3 require slightly more explanation.

Preparing Boxplots in Excel The boundary between the second and third quartile, Q2, is what we call the median. Technically speaking, the median is the point at which equal amounts of data are on either side. If the number of data items is odd, we simply pick the median to be the middle item. If the number of data items is even then we take the median to be the average of the middle two items. Of course the data needs to be sorted in ascending order first.

Preparing Boxplots in Excel Let us try an example. Find the median for the following two sets of data, which we will also use as the data for our five number summary and boxplots. (Note that we are using two data sets as the technique used to do boxplots in Excel requires at least two data sets.) 14, 16, 17, 19, 23, 25, 27 8.3, 9.4, 9.8, 11.2, 13.1, 13.9, 14.7, 16.5

Preparing Boxplots in Excel The median is defined as the middle data item if there are an odd number of items, and the average of the middle two items if there are an even number of items. Data set 1 has 7 values, so the middle value is value 4, or 19. Data set 2 has 8 values, so the middle two values are values 4 and 5, ie 11.2 and 13.1. The average of these is 12.15. Hence the medians for our two data sets are 19 and 12.15 Note that the sets were ordered before calculating.

Preparing Boxplots in Excel What about the quartile boundaries Q1 and Q3? Q1 is defined to be the median of the data from Q0 to Q2, including Q2. Q3 is defined to be the median of the data from Q2 to Q4, including Q2. Hence for set 1, we seek Q1 = the median of 14, 16, 17, 19, ie the average of 16 and 17, or 17.5. Also Q2=the median of 19, 23, 25, 27, ie the average of 23 and 25, or 24. Hence Q0=14, Q1=17.5, Q2=19, Q3=24, Q4=27 for data set 1.

Preparing Boxplots in Excel For set 2, Q1=median of 8.3, 9.4, 9.8, 11.2, 12.15. There are 5 values, so we accept the third one as Q1, ie Q1=9.8. Q3=median of 12.15, 13.1, 13.9, 14.7, 16.5 or 13.9. Hence Q0=8.3, Q1=9.8, Q2=12.15, Q3=13.9, Q4=16.5. Now to prepare the boxplots for our two data sets.

Preparing Boxplots in Excel First we need to set up a spreadsheet with the five number summaries. Notice the unusual order of the quartile boundaries.

Preparing Boxplots in Excel Select the data, including the labels, and click the Chart Wizard.

Preparing Boxplots in Excel Choose a line chart.

Preparing Boxplots in Excel Click next and check the Rows box, to accept data in rows instead of columns.

Preparing Boxplots in Excel Click Next and give the chart a title.

Preparing Boxplots in Excel Click Finish and the chart will be inserted into your worksheet.

Preparing Boxplots in Excel At this point the chart looks nothing like a boxplot. There are only a few steps left. Select each of the lines in turn, by right clicking and choose the Format Data Series option. Select the Patterns tab and turn the line off.

Preparing Boxplots in Excel Remove all five lines.

Preparing Boxplots in Excel

Preparing Boxplots in Excel Now the graph consists only of a set of isolated points.

Preparing Boxplots in Excel Right click any of the data points and choose the Format Data Series option, and choose the Options tab. Select the High-Low lines and the Up-Down bars.

Preparing Boxplots in Excel

Preparing Boxplots in Excel Click OK and your boxplots will be ready to be resized, moved and printed.