Section author: Danielle J. Navarro and David R. Foxcroft


Let’s begin with the humble histogram. Histograms are one of the simplest and most useful ways of visualising data. They make most sense when you have an interval or ratio scale variable (e.g., the afl.margins variable from the aflsmall_finalists data set that we used in Descriptive statistics) and what you want to do is get an overall impression of the variable. Most of you probably know how histograms work, since they’re so widely used, but for the sake of completeness I’ll describe them. All you do is divide up the possible values into bins and then count the number of observations that fall within each bin. This count is referred to as the frequency or density of the bin and is displayed as a vertical bar. The afl.margins variable contains 33 games in which the winning margin was less than 10 points and it is this fact that is represented by the height of the leftmost bar that we showed earlier in Descriptive statistics, and Fig. 20. With these earlier graphs we used an advanced plotting package in R which, for now, is beyond the capability of jamovi. But jamovi gets us close, and drawing this histogram in jamovi is pretty straightforward. Open up the Plots options under ExplorationDescriptives and click the Histogram check box, as shown in Fig. 21. jamovi defaults to labelling the y-axis as density and the x-axis with the variable name. The bins are selected automatically, and there is no scale, or count, information on the y-axis unlike the previous Fig. 20. But this does not matter too much because after all what we are really interested in is our impression of the shape of the distribution: is it normally distributed or is there a skew or kurtosis? Our first impressions of these characteristics come from drawing a histogram.

Histogram check box in jamovi

Fig. 21 jamovi screen showing the histogram check box

One additional feature that jamovi provides is the ability to plot a density curve. You can do this by clicking the Density check box under the Plots options (and unchecking Histogram), and this gives us the plot shown in Fig. 22. A density plot visualises the distribution of data over a continuous interval or time period. This chart is a variation of a histogram that uses kernel smoothing to plot values, allowing for smoother distributions by smoothing out the noise. The peaks of a density plot help display where values are concentrated over the interval. An advantage density plots have over histograms is that they are better at determining the distribution shape because they’re not affected by the number of bins used (each bar used in a typical histogram). A histogram comprising of only 4 bins wouldn’t produce a distinguishable enough shape of distribution as a 20-bin histogram would. However, with density plots, this isn’t an issue.

Density plot for the ``afl.margins`` variable

Fig. 22 Density plot for the afl.margins variable plotted in jamovi

Although this image would need a lot of cleaning up in order to make a good presentation graphic (i.e., one you’d include in a report), it nevertheless does a pretty good job of describing the data. In fact, the big strength of a histogram or density plot is that (properly used) it does show the entire spread of the data, so you can get a pretty good sense about what it looks like. The downside to histograms is that they aren’t very compact. Unlike some of the other plots I’ll talk about it’s hard to cram 20-30 histograms into a single image without overwhelming the viewer. And of course, if your data are nominal scale nominal then histograms are useless.