Plotting cumulative frequency at class midpoints
"The Midpoint Mistake"
The Mistake in Action
For the class 20-30, plot the cumulative frequency at x = 25.
Wrong: Point plotted at (25, CF)
Why It Happens
Students are used to plotting at midpoints for frequency polygons and histograms, so they apply the same rule here.
The Fix
For cumulative frequency, plot at the upper class boundary.
The class 20-30 has upper boundary 30. Plot the cumulative frequency at x = 30, not 25.
Why? Cumulative frequency tells you how many values are less than or equal to a value. By the end of the class 20-30, you've counted everyone up to 30.
Spot the Mistake
Can you identify where this student went wrong?
Class 20-30
Plot at x = 25
Click on the line that contains the error.
Related Topics
Learn more about the underlying maths: