6.- The harmonic mean (see above) is the appropriate method for averaging ratios consisting of numerators and denominators (unemployment rates, sales productivity, miles per hour, price per gallon, people per square feet, etc.) when the values in the numerator are not identical. Consider, for instance, the sales productivity (as measured in revenue per employee) of three companies with differing headcounts but identical revenues: Company Sales Employees SPi Note: SP = Sales per Employee. 1 1000 10 100 SPi = Sales per Employee at 2 1000 5 200 the ith company. 3 1000 2 500 ni = Number of employes in Sum = 800 = (SPi) each company. n = (ni) = 10+5+2 = sum