Choice of class mark
The class mark is the midpoint of a class interval and is used as the representative value of that class in grouped-data calculations.
Practice This ConceptMain explanation
Teacher explanation
The class mark stands for the centre of a class interval. It is found by adding the lower and upper limits and dividing by 2. In mean calculations, it provides one value that can represent all observations inside the class.
Example
For the class 20-30, the class mark is (20 + 30) / 2 = 25.
Simple analogy
Middle of the interval, middle of the calculation.
Common confusion
Students sometimes choose the upper limit or lower limit instead of the midpoint.
Exam tip
If the class interval is written as a range, the midpoint is almost always the safest representative value.
Answer writing and exam use
1-mark use
Write the exact meaning of choice of class mark in one clean line.
2-mark use
Define choice of class mark and add one example or condition.
3-mark use
Explain choice of class mark, show the method or example, and mention the common mistake.
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