Editing Consumer price index (section)

===Reweighting===
Ideally, in computing an index, the weights would represent current annual expenditure patterns. In practice, they necessarily reflect past using the most recent data available or, if they are not of high quality, some average of the data for more than one previous year. Some countries have used a three-year average in recognition of the fact that household survey estimates are of poor quality. In some cases, some of the data sources used may not be available annually, in which case some of the weights for lower-level aggregates within higher-level aggregates are based on older data than the higher level weights.

Infrequent reweighting saves costs for the national statistical office but delays the introduction into the index of new types of expenditure. For example, subscriptions for Internet service entered index compilation with a considerable time lag in some countries, and account could be taken of digital camera prices between re-weightings only by including some digital cameras in the same elementary aggregate as film cameras.