Grouped data refers to data on a continuous variable, such as weight, that has been divided into segments. For instance, for weights of adult women the groups might be 80 to 99 pounds, 100 to 119 pounds, 120 to 139 pounds, and so on. The mean is the proper statistical name for the average.
Note that the groups do not have to be the same size -- in the example, most groups covered 20 pounds but a heaviest group could cover 50 pounds. However, if one of the groups is "or less" or "or more" then you must make strong assumptions to calculate the mean.
Calculate the midpoint of each group. This is simply the average of the lowest and highest values in the group. In the example above, the midpoints are 89.5 pounds, 109.5 pounds and 129.5 pounds.
Multiply the number of subjects in each group by the group midpoint.
Add up the products from Step 2.
Divide the total by the number of subjects. This is the approximate mean.