Some purchasing specialist jobs also assign specialists to track the maintenance of office equipment. Maintaining records of warranties and service agreements is sometimes expected of a person in this position.
A retail price index would take this into account and use a weighted average which gives more prominence to commonly bought items. Excel does not contain a built in function to calculate a weighted average. It is however easy to do it using the SUMPRODUCT() function in a simple formula. SumProduct() multiplies two arrays (or ranges) together and returns the sum of the product. In the illustration it would calculate '(B4 x C4) + (B5 x C5) + (B6 x C6)'.
For example, the weighted average of a list of numbers $x_1,\ldots,x_n$ with corresponding weights $w_1,\ldots,w_n$ is $${w_1\cdot x_1+w_2\cdot x_2+\ldots+w_n\cdot x_n\over w_1+w_2+\ldots+w_n}$$ Note that if the weights are all just $1$, then the weighted average is just a plain average.