Calculation of value indice:

The gross turnover indices are calculated at the most detailed level of the French classification of activities : the NAF rev. 2 sub-classes. At this level, the index for month m is calculated by linking the index for month m-12 to the change in turnover between m-12 and m. Business demographics (start-ups, closures, entries and exits, changes in sector of activity) are taken into account on a continuous basis to accurately reflect the economic reality. Restructurings that may affect the sectoral outline of VAT returns are treated separately.

The turnover indices change their base and reference every 5 years, except for the current reference year being 2021 in reason of the particularities of year 2020. The indices now published have a base and reference year of 2021. The average value of the indices is 100 in 2021 (previously 100 in 2015).

Calculation of indices in volume:

The sales volume indices at a fine level are defined as the ratio of the turnover index in value to the associated price deflator. This is done for the 116 sub-classes in the trade sector. Once the indices at the finest level of nomenclature are calculated, the indices for higher levels are calculated by aggregation (cf infra). They are disseminated on the INSEE website from the class level. They are calculated since 2005, according to the availability of the prices.

In Retail trade and Trade and repair of motor vehicles and motorcycles price indices used as deflators are obtained by aggregating the Consumer Price Indices (CPIs) of the products concerned. A cross-reference table is established between the sectors of the Naf rev 2 and the consumption classification (Classification of Individual Consumption by Purpose - COICOP).

The Consumer Price Index (CPI) is measured all taxes included, so it is necessary to neutralize VAT rate changes.

For sales volume indices in wholesale trade, different price indices are used according to the products: IPAMPA (Monthly agricultural means of production purchasing price indices), IPPAP (Monthly agricultural producer price indices), IPPI (French industry producer price indices) and consumer price indices. The sectoral price index is therefore a weighted average of the various indices representing the products making up the sales structure.

Aggregation to upper levels:

From the most aggregated level (i.e. on wholesale and retail trade) down to level A 129 of the french aggregated classification (https://www.insee.fr/fr/information/2028155), the calculation of weights is based first and foremost on national accounts data on production sold, broken down by branch, at basic prices. By definition, output at basic prices excludes taxes on products, but is increased by subsidies on products; it does not include transport services. At finer levels of classification, the estimation of weights is based on INSEE's Esane statistics (structural business statistics).

The previous 2015 rebasing implemented an innovation, with the introduction of a two-weighting system (instead of a constant-weight system) in order to better take into account the structural evolutions. With the current 2021 rebasing, there are 3 different weights. 2010 weights are used for the aggregation of elementary indices between 2005 (respectively 1999 for turnover indices) and 2012, 2015 weights are used between 2013 and 2017, and 2021 weights are used from 2018. The calculation on the whole period is then achieved by chaining sub-series (computation of a link coefficient on the years 2013 and 2018).