Calculating the wrong class width H
"The Width Blunder"
The Mistake in Action
Calculate the frequency density for this class: Mass: 50 ≤ m < 75, Frequency: 20
Wrong: Width = 75 + 50 = 125 FD = $20 ÷ 125 = 0.16$
Why It Happens
Students add the boundaries instead of subtracting, or make errors reading the class boundaries.
The Fix
Class width = Upper boundary − Lower boundary
For the class 50 ≤ m < 75: $$\text{Width} = 75 - 50 = 25$$ $$\text{FD} = \frac{20}{25} = 0.8$$
Watch out for different notations:
- "50-75" usually means 50 ≤ m < 75 (width = 25)
- "50-" followed by "75-" means the classes are 50 ≤ m < 75 and 75 ≤ m < (next value)
Spot the Mistake
Can you identify where this student went wrong?
Mass: 50 ≤ m < 75, Frequency: 20
Width = 75 + 50 = 125
Click on the line that contains the error.
Related Topics
Learn more about the underlying maths: