For example , suppose in the above example , there are three types of students : 1 ) Below normal , if the response time is 5 to 14 seconds , 2 ) normal if it is between 15 and 24 seconds , and 3 ) above normal if it is 25 seconds or more , then the grouped data looks like : Table 3 : Frequency distribution of the three types of students An estimate , , of the mean of the population from which the data are drawn can be calculated from the grouped data as : In this formula , x refers to the midpoint of the class intervals , and f is the class frequency .
Suppose there are three types of students : then the grouped data looks like the following : Table 3 : Frequency distribution of the three types of students An estimate , , of the mean can be calculated from grouped data .