The information thus collected is reduced on the same date for all municipalities to ensure equal treatment between them and to get good data reliability.

In municipalities with 10,000 inhabitants or more, this calculation is based on an exhaustive address control list (named "RIL"), which gives the number of households at each address.

In municipalities with less than 10,000 inhabitants, the evolutions of the numbers of households according to the local residence tax (TH) files are used.