The class mark
A grouped table does not show the single readings. Each row is only a span.
Step 1 of 3
- Five classes, one row each.
- The class 10-20 highlighted, its mark at 15.
- Thirteen actual readings plotted — none on 15.
One number for a class
A row like 10-20 does not say which readings are inside it. So we pick one number to represent the whole class. The middle is the fairest guess when we cannot see readings. This number is the class mark, the middle of the class.
| Minutes played | Frequency | Class mark |
|---|---|---|
| 0-10 | 7 | 5 |
| 10-20 | 13 | 15 |
| 20-30 | 10 | 25 |
| 30-40 | 6 | 35 |
| 40-50 | 4 | 45 |
For 10-20 the class mark is (10 + 20) ÷ 2 = 15. The chapter makes an assumption. Each class's readings are spread evenly around its mark.
The class mark is the two bounds added, then halved. It stands in for the class. No reading has to equal it.
Example: the class 65-85 has mark (65 + 85) ÷ 2 = 75. The class width is 20, not 75.
Tap the class mark for four classes.
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