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Histograms vs. Bar Graphs Discussion
Student: Histograms and bar graphs look very similar. What's different about them?
Mentor: I think of histograms as "sorting bins."
You
have one variable, and you sort data by this variable by placing them into "bins." Then you count how many pieces of data are in each bin. The height of the rectangle you draw on top of each bin is proportional to the number of pieces in that bin.
On the other hand, in bar
graphs
you have several measurements of different items, and you compare them. The main question a histogram answers is: "How many measurements are there in each of the classes of measurements?" The main question a bar graph answers is: "What is the measurement for each item?" Here are some examples:
Situation |
Bar Graph or Histogram? |
| We want to compare total revenues of five different companies. |
Bar graph. Key question: What is the revenue for each company? |
| We have measured revenues of several companies. We want to compare numbers of companies that make from 0 to 10,000; from 10,000 to 20,000; from 20,000 to 30,000 and so on. |
Histogram. Key question: How many companies are there in each class of revenues? |
| We want to compare heights of ten oak trees in a city park. |
Bar graph. Key question: What is the height of each tree? |
| We have measured several trees in a city park. We want to compare numbers of trees that are from 0 to 5 meters high; from 5 to 10; from 10 to 15 and so on. |
Histogram. Key question: How many trees are there in each class of heights? |
Student: I can find lots of such examples in newspapers or magazines.
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