The volume of sales index, more commonly called the index of the volume of (retail) sales, is the value of retail sales in terms of its volume.
The volume of sales index is a volume measure of the retail trade turnover index. In order to eliminate the price effect on turnover in the retail trade, a deflator of sales is used. This sales deflator is an index with a similar methodology to that of an output price index but it is adapted specifically for the retail trade. It reflects price changes in the goods sold rather than the sales service provided.
It should be noted that the volume of sales is different from the volume of retail trade services. The latter takes into account changes in the quality of the trade service supplied. As such, the sales volume is conceptually different from the production index which takes into account quality changes.