As already discussed under the head, types of index numbers, a quantity, or volume index number is one that measures the changes in the level of quantities consumed during a given period with reference to a base period. Just like the price indices the quantity indices change constructed under the various methods explained above. The only thing to do here is to reverse the position of P and q and the quantities are treated as the variables. Thus, the various formula discussed above will have to be modified as under for constructing a quantity index number.