While computing mean of grouped data, we assume that the frequencies are
evenly distributed over all the classes
centred at the class marks of the classes
centred at the upper limits of the classes
centred at the lower limits of the classes
Step 1: In grouped data, values are arranged in class intervals (like 0–10, 10–20, etc.).
Step 2: Inside each interval, we do not know the exact data values of all items. For example, in the class 0–10, we only know how many values (frequency) are there, but not the exact numbers.
Step 3: To calculate the mean, we need to assume one representative value for each class interval.
Step 4: The most fair representative value of a class is its class mark, which is the midpoint of the interval. For example, for 0–10, the class mark is (0+10)/2 = 5.
Step 5: So, we assume that all the data values (frequency) in that class are concentrated at this midpoint.
Step 6: Therefore, when we compute mean of grouped data, we take frequencies as if they are centred at the class marks.
Answer: Option (B) – centred at the class marks of the classes.