What Are Histograms?
When we collect data, we often end up with a large set of numbers that can be hard to interpret. Histograms are one of the most powerful ways to visualise continuous data that has been grouped into intervals (also called classes or bins).
A type of bar chart used to display the frequency distribution of continuous data. The bars are drawn with no gaps between them, and the area of each bar represents the frequency of data in that interval.
Histograms might look like bar charts at first glance, but there's a crucial difference:
- Bar charts are used for categorical (discrete) data — like favourite colours or types of pet. The bars have gaps between them.
- Histograms are used for continuous (numerical) data — like heights, weights, or time. The bars touch because the data flows continuously from one interval to the next.
Think of a histogram like a row of lockers along a hallway — each locker (bar) holds a range of values, and they sit right next to each other with no gaps because the number line has no gaps either.
Key Features of a Histogram
Every well-constructed histogram has the following features:
- Title — describes what the histogram represents
- Horizontal axis (x-axis) — shows the continuous variable, divided into class intervals (e.g., 0–10, 10–20, 20–30)
- Vertical axis (y-axis) — shows frequency (the number of data values in each interval) or frequency density when intervals have unequal widths
- Bars — each bar spans the width of its class interval, and bars are drawn touching each other
- Axis labels — both axes must be clearly labelled with units
A range of values into which continuous data is grouped. For example, if we measure heights of students, a class interval might be cm.
A very common mistake is leaving gaps between the bars in a histogram. Remember: no gaps — the data is continuous! Gaps would imply that some values are impossible, which isn't the case for continuous data.
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