The price series of the control benefits are transformed into price indices, called "elementary series". These elementary series are then themselves aggregated into indices corresponding to items in the NAF nomenclature, called "basic series", most often NAF subclasses (separately between residential and non-residential). The weights of the elementary series used for this aggregation are those obtained from the legal unit during the investigator's visit and are therefore fixed throughout the life of the control product. If, for a given quarter, the price of a service cannot be observed, the change in this price is estimated by the weighted average of the price changes of services belonging to the same class (subclass) of services.
The basic series are then themselves aggregated into higher-level indices, called "synthetic series": aggregation of NAF5 indices into NAF4 indices, aggregation of NAF4 indices into total IPEA indices, aggregation of residential and non-residential indices into all-buildings indices.
The weights of the items in the nomenclature used for these aggregations are changed each year, so that the published IPEA indices correspond to annually chained Laspeyres indices since that date.
More precisely, the weights of the items in the nomenclature for an A year correspond to the turnover for year A-2 observed in the annual structural survey. By exception, the weightings for the years 2013 to 2017 correspond to sales for year A-1. Until 2013, the indices are Laspeyres fixed base indices.
For each work class, the non-residential IPEA index is calculated by aggregating elementary series over the data availability period and back-polished until the 1st quarter of 2015 from the residential index for the same work class.
The indices are expressed in reference 100 in 2015. This means that, by construction, their average over the four quarters of the year 2015 is 100.