This tutorial shows how to draw and interpret notched box plots in Excel using the XLSTAT software.
Dataset for drawing a notched box plot
An Excel sheet with both the data and the results can be downloaded by clicking here.
A description of the dataset as well as a guide to interpret classic box plots is available in the XLSTAT classic box plots tutorial.
In this tutorial, we add the notched option to the Box plots that are generated in the box plots tutorial. Notches allow to visualize a form of confidence intervals around the medians. When sample sizes are similar, notches allow to say roughly if medians are different or not.
Setting up the dialog box for the notched box plots
Once XLSTAT is open, select the XLSTAT / Visualizing data / Univariate plots command, or click on the corresponding button of the Visualizing data toolbar (see below).
Once you have clicked on the button, the Descriptive Statistics dialog box appears.
Follow all the steps that are described in the classical box plots tutorial, but make sure that the notched option is activated in the Charts (1) / Options tab.
Interpreting notched box plots
The results are displayed on the new sheet named "Desc".
After a table summarizing descriptive statistics, the notched box plots appear.
One box plot per species is displayed.
The red crosses correspond to the means. The central horizontal bars are the medians. Notches limits represent confidence intervals around medians. As notched areas do not overlap across box plots, we may expect that the three medians are different from one another. Please refer to Mood’s test to test for the null hypothesis of median equality. Box (vertical) thickness represents the InterQuartile Range (IQR) and allows to visualize the dispersion of 50% of the central data points around the median.