When data is in grouped classes, you cannot find the exact mean. Instead, you use the midpoint of each class as an estimate. This gives an estimated mean.
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Key facts to remember
1Use the midpoint of each class interval as the representative value.
2Midpoint = (lower class boundary + upper class boundary) ÷ 2.
3Estimated mean = Σ(midpoint × frequency) ÷ Σfrequency.
4The modal class is the class with the highest frequency.
5The median class is found using the cumulative frequency — locate the n/2 th value.
6Answers are estimates because the exact data values are unknown.