In order to synthesise the main characteristics, installations and services of dwellings, each installation or service and the selected characteristics were classified into 3 categories: basic, medium or high level.

This classification provides an index whose value is a good indication of the level of comfort or well-being that a dwelling may have or provide.

The scores assigned to main family dwellings were as follows:

Basic level amenities:

- Electricity (in all cases) ....1
- Kitchen (in all cases) ....1
- Running water (in all cases) ....1
- Dwelling built between 1960 and 1990 ....1
- No heating but with a mobile or fixed device to raise the temperature ....1
- No central hot water (but does have hot water) ....1
- No Internet or telephone in the dwelling (landline or mobile) but the building has a telephone line ....1
- Between 19.4 and 45.8m2 per person ....1
- Between 0.28 and 1.05 people per room ....1
- No lift but the entrance hall is accessible ....1

Medium level amenities:

- Dwelling built after 1990 ....2
- Heating ....2
- Central hot water ....2
- Only 1 bathroom ....2
- Gas supplied by pipe in the building but not in the dwelling ....2
- A telephone in the dwelling (landline or mobile) but no Internet ....2
- More than 45.8m2 per person ....2
- Less than 0.28 people per room ....2
- Has a lift but the entrance hall is not accessible ....2
- Garage ....2

High level amenities:

- 2 or more bathrooms ....3
- Gas supplied by pipe in the dwelling ....3
- Internet ....3
- Has a lift and the entrance hall is accessible ....3

According to these scores, the maximum value that a dwelling can have is 27. To facilitate the processing of the index, the scores assigned were converted to a scale of 100.

We consider the dwelling to have a low comfort level when the index is below 50 points, medium comfort between 50 and 75, and high comfort above 75.