Sampling for the VASyR follows a two-stage cluster approach, keeping with the methodology of previous years. UNHCR database of known Syrian refugees as of June 2020 served as the sample frame. Cases with missing addresses were excluded. Sampling was based on a "30 x 7" two-stage cluster scheme initially developed by the World Health Organization. This method outlines a sample size of 30 clusters per geographical area and seven households per cluster which provides a precision of +/- 10 percentage points. Districts were considered as the geographical level within which 30 clusters were selected. There are 26 districts in Lebanon, where Beirut and Akkar each represent a district and a governorate. As such, to ensure similar representativeness with other governorates, an additional two cluster samples were considered for each, yielding 90 cluster selections for each. The governorate of Baalbek Hermel is made up of only two districts, as such, and to ensure an adequate sample in that governorate, one additional cluster sample was considered.
The primary sampling unit was defined as the village level (i.e. cluster) and UNHCR cases served as the secondary sampling unit. A case was defined as a group of people who are identified together as one unit (usually immediate family/household) under UNHCR databases. Using Emergency Nutrition Assesment (ENA) software, villages were selected using probability proportionate to size where villages with a larger concentration of refugees was more likely to be selected and 30 clusters/villages were selected with four replacement clusters, per district.
In order to estimate the sample size needed to generate results that are representative on a district, governorate and national level, the following assumptions were used:
- 50% estimated prevalence
- 10% precision
- 1.5 design effect
- 5% margin of error
Using the above parameters, 165 cases per district/cluster selection was required, leading to a target of 5,115 cases nationally. Due to the known high level of mobility of the Syrian refugee population and based on experience in previous rounds of VASyR and other household level surveys, a 40% non-response rate was considered.
