In order to prepare for the dissemination of the 1999 population census, INSEE developed a system for dividing the country into units of equal size, known as IRIS2000. In French, IRIS is an acronym of ‘aggregated units for statistical information’, and the 2000 refers to the target size of 2000 residents per basic unit.
Since that time IRIS (the term which has replaced IRIS2000) has represented the fundamental unit for dissemination of infra-municipal data. These units must respect geographic and demographic criteria and have borders which are clearly identifiable and stable in the long term.
Towns with more than 10,000 inhabitants, and a large proportion of towns with between 5,000 and 10,000 inhabitants, are divided into several IRIS units. This separation represents a division of the territory. France is composed of around 16,100 IRIS, of which 650 are in the overseas departments.
By extension, in order to cover the whole of the country, all towns not divided into IRIS units constitute IRIS units in themselves.
There are 3 types of IRIS unit:
- the residential IRIS: population generally falls between 1,800 and 5,000. The unit is homogeneous in terms of living environment and the boundaries of the unit are based on the major dividing lines provided by the urban fabric (main roads, railways, bodies of water etc.)
- the business IRIS: containing more than 1,000 employees, with at least twice as many employees as other residents.
- the miscellaneous IRIS units: specific large zones which are sparsely inhabited and have a large surface area (leisure parks, ports, forests etc.).
As of January 1st 2008, 92% of IRIS units were residential, with 5% business. Since their creation, the demographic characteristics of certain IRIS units may have evolved, although their classification will not have been updated.
In 2008 a very partial reworking of the division system was undertaken to take into account major developments in the road network or demographics. This reworking was limited to around 100 IRIS units, in order to preserve continuity in the data publication series.
Division of a territory into IRIS units may be affected by modifications in the geography of the municipalities (merging of towns or villages, founding or repopulation of municipalities, land exchanges). So it is useful to specify the year of reference, for example by noting either IRIS-geography 1999 or IRIS-geography 2008.
A TRIRIS is a grouping of IRIS (generally 3 IRIS).
The TRIRIS was created in 1999 for dissemination of sensitive variables of the census for which the IRIS seems insufficient to guarantee the statistical secret.
Its code, on 6 positions, consists in the department code and in an order number on 3 positions (the last position is an indicator of TRIRIS). The code belongs to ZZZZZZ if the municipality is not cut in IRIS or if IRIS are not included in TRIRIS.