Weighted arithmetic mean assigns different weights or levels of importance to values before calculating the average. It is calculated by multiplying each value by its weight, summing these products, and dividing by the total of the weights. Combined arithmetic mean calculates the average of two or more groups by taking the weighted average of the means of each group, where the weights are the number of items in each group. Weighted and combined arithmetic means are useful when not all values have equal importance or when calculating averages of related subgroups.