Table 1 Overview of the sampling scheme of the Belgian health interview survey

Overall methodological approach of the BHIS

The aim of the survey is to realise a prefixed number of interviews in every region per quarter. A methodology is used in which groups of 50 individuals (in a number of selected households) will be interviewed. The number of groups equals the prefixed number of interviews in every region divided by 50. In each quarter on average 12.5 individuals per group are to be interviewed. The number of groups to be considered in every province (within every region) is proportional to the number of inhabitants of the provinces.

Step 1: selecting municipalities

To determine in which municipalities the groups of individuals will be selected, municipalities are ordered within every province according to their size (number of inhabitants). A systematic selection procedure is used (based on a random start and an interval equal to the size of the province divided by the number of groups to be selected in the province) to attribute groups to municipalities within the provinces. It is possible that several groups are selected in the same large municipality.

Step 2: selecting households

Within every selected municipality, households are ordered hierarchically by:

- statistical sector

- the size of the household in 5 categories : size 1, 2, 3, 4, and 4+

- the age of the reference person

The number of households to be sampled per quarter is theoretically 12,5 divided by the average size of the households of the selected municipality. In order to have enough substitute-households the numerator doubled (25 instead of 12,5). For this calculation, the size of household with more than 4 members is recoded as 4 (because only a maximum of 4 members per household can be selected for the interview).

The step-size (or 'interval’) used to select the household is defined as the number of households within the municipality divided by the number of household to be sampled in the municipality.

For every selected household during the sampling, three consecutive households in the order are selected, this in the context of substituting non-participating households. Such quadruples of households are called “clusters”.

To prevent any order effect, the households within each cluster are randomized, while the clusters themselves are randomised too. After applying this procedure, the fieldwork starts using the first ranked cluster/the first ranked household within the cluster and working from the top to the bottom of the list until the prefixed number of interviews is achieved.

Step 3: selecting individuals

In participating households, a maximum of 4 members are selected for the interview: the reference person, the partner (if present) and 3 (no partner) or 2 (partner present) other random selected household members. For non-participating households, substitute households are activated. This process continues till the regional prefixed number of interviews is attainted.