The sales-value-at-split-off method can be approximated through the use of weighting factors based on price. The advantage is that the price-based weights do not change as market prices do. An example of this method is found in the glue industry. Material is put into process in the cooking Department. The products resulting from the cooking operations are the several "runs of glue." The first run is of the highest grade, has the highest market value, and cost the least. Successive runs require higher temperatures, cost more, and produce lower grades of products. Glue factories do not attempt to determine the actual cost of each skimming because the effect would be to show the lowest cost on the first grade of product and the highest cost on the lowest grade. Instead, the cost of all glue produced is determined, and this total cost is spread over the various grades on the basis of their respective tests of purity. The relative degree of purity is an indicator of the quality and, therefore, of the market value of each run or grade produced. Hence, multiplying the yield for each run by its relative purity is equivalent to multiplying it by the market value. The amounts weighted by purity are used to allocate the joint costs to each run. Additional runs would be undertaken, of course, only as long as the incremental revenue of the additional run is equal to or exceeds the incremental costs incurred.