Chapter 4: Use of Weights to combine Data across Countries and Rounds
Most users of ESS data will want results based on samples that represent the country or group of countries they are interested in as closely as possible. This means that the most accurate estimates will only be obtained after first weighting the data.
The two weights present in each round of the ESS are the Design weight (DWEIGHT) and the Population size weight (PWEIGHT):
Several of the sample designs used by countries participating in the ESS were not able to give all individuals in the population aged 15+ precisely the same chance of selection. Thus the unweighted samples in some countries over or under-represent people in certain types of address or household, such as those in larger households. The design weight corrects for these slightly different probabilities of selection.
This weight corrects for the fact that most countries taking part in the ESS have very similar sample sizes, no matter how large their population. Without weighting, any figures combining two or more countries’ data would be incorrect, over-representing smaller countries at the expense of larger ones. The Population size weight makes an adjustment to ensure that each country is represented in proportion to its population size.
These two weights should almost always be used when analysing ESS data. Furthermore, when combining data from several rounds, further measures need to be taken. This chapter will describe in detail when and how the weights should be used.