Unit 21 · Bar Diagrams
Bar Diagrams
Mathematics · Grade 7
What You Will Be Able To Do
📊Draw a bar diagram properly, with scale, labels and equal widths
👥Use a double bar diagram to compare two related sets
🚧Tell a bar diagram from a histogram, and why the gaps matter
🔍Spot a chart that misleads while every printed number is correct
“
A bar diagram works because human beings judge length very well and judge numbers very badly. The reader is comparing lengths, so the lengths must be in proportion to the numbers.
Presenter notes: Open with the same forty marks from last lesson, first as a table and then as a bar chart, and ask which one they understood faster. The answer is obvious and it is worth saying why: people compare lengths very well and compare numbers very badly. Then warn them that this strength is also a weakness, because a chart can exploit it, and by the end of the lesson they will know exactly how.