Measures of Dispersion for Grouped Data
Class interval
A range of values into which grouped data is sorted.
| English | Class interval |
|---|---|
| Bahasa Melayu | Selang kelas |
| 中文 | 组距 |
How it is used
Marks grouped as 1–10, 11–20, 21–30 each form a class interval of width 10. The interval 21–30 has lower boundary 20.5 and upper boundary 30.5, so its true width is 30.5 − 20.5 = 10.
Where it shows up in SPM
In grouped-data statistics, Form 5. Class intervals are how continuous data is sorted before any calculation.
In Paper 2 you use them to find class midpoints for the mean, boundaries for the ogive, and to identify the modal class, so reading the intervals correctly underpins the whole question.
Don't confuse it with
Open the chapter: Measures of Dispersion for Grouped Data →
Frequently asked questions
How do I turn a class interval into boundaries?
Take the gap between one class and the next and split it evenly. Between 1–10 and 11–20 the gap sits at 10.5, so 1–10 has boundaries 0.5 and 10.5.
Subtract 0.5 from the lower limit and add 0.5 to the upper limit.
Do all class intervals have to be the same width?
In the SPM Mathematics grouped-data questions they are normally equal, which keeps the mean and ogive calculations straightforward. Equal widths also let a histogram use bar height directly for frequency, since every bar covers the same interval.