Statistics students often represent raw observations in a grouped frequency table before finding averages.
Answer the following questions related to this process.
(i) For the class interval 60 – 80, write the value of its class mark.
(ii) Give one clear reason why class marks are used instead of individual class limits while calculating the arithmetic mean of grouped data.
(iii) A student tried to find the mean of a distribution by simply adding the lower limits of all classes and dividing by the number of classes. Identify the conceptual error in this approach and briefly describe the correct procedure in not more than three ordered steps.