The least accurate method of calculating cost new and the least used method is the index method (used mainly to corroborate cost estimates from other methods). The index method attempts to calculate the cost new of a structure by multiplying its original cost by the change in some known index associated with price increases. While the cost approach is normally not concerned with the original or historic cost of a structure, the index method requires the original cost, year built, and cost index from the year of construction to be known. It is also necessary to know the current year index. The index would be associated with price increases in construction and could be consumer price index or construction cost index. To calculate the cost new using the index method, multiply the original cost of the structure times the new index over the old index (the new index over the old index reflects the price changes during the period of time).