Page "Consumer price index" Paragraph 17

Most elementary aggregate indexes are necessarily ' unweighted ' averages for the sample of products within the sampled outlets.

However, in cases where it is possible to select the sample of outlets from which prices are collected so as to reflect the shares of sales to consumers of the different outlet types covered, self-weighted elementary aggregate indexes may be computed.

Similarly, if the market shares of the different types of product represented by product types are known, even only approximately, the number of observed products to be priced for each of them can be made proportional to those shares.