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)'.
Solving this equation for y, which is the unknown value at x, gives. which is the formula for linear interpolation in the interval ( x 0 , x 1 ) {\displaystyle (x_{0},x_{1})} . Outside this interval, the formula is identical to linear extrapolation. This formula can also be understood as a weighted average.