This is the fourth in a series of tips on different chart types. The idea is to fill up your toolbox with a variety of charts for making sense of data. This week, I give you the histogram.
Active Ingredients (What is a histogram?)
Histograms look like bar charts. Along the X-axis are “bins,” which each represent a range of values. The Y-axis shows how many units fall into each bin (as long as bins are equally spaced. See warning below.)
Uses
A quick look at a histogram shows you where values are concentrated, what the highest and lowest values are, and whether there are any gaps or unusual values. As such, histograms provide a snapshot of the shape and distribution of the data. Histograms are a great way to get to know your data and give you a much clearer picture than a simple average of your values would.
The series of histograms below provides a snapshot of how 87 students are doing in school. We can see that:
Absences, referrals*, and F grades are all relatively rare. A lot of kids fall into the zero bin.
Although missing assignments were also less common, most kids had some.
Reading and math test scores (MAP) were more “normally distributed” meaning lots of scores concentrated in the middle of the range, with the remaining scores trailing off symmetrically on both sides.
Source: http://www.storytellingwithdata.com/blog/2019/2/21/various-views-of-variability
Warnings
If you make your bins too big or too small, it will be difficult to see the underlying pattern of the data. There’s no rule of thumb about how to size your bins. Play around with it to see what makes the overall pattern clear.
Technically, histograms are based on the area, not the height, of bars. The height of the bar does not necessarily show how many units there are within each bin. Instead, the height times the width of the bin gives you the number of units in the bin. However, most histograms that I run across have a standard sized bin, and under these circumstances, the height of the bin does reflect the number of units.
Fun Fact
Histogram = histos (Greek for mast) + gram (Greek for something written or recorded). So maybe the term was applied because the chart looks like a row of masts.
*I assume this means discipline referrals.
To see past data tips, including those about other chart types, click HERE.
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