European Social Survey 2018, Round 9

There are three methods of weighting that were used in the study. A summary is provided below.

• Design Weights: Individuals in the population aged 15+ have precisely the same chance of selection. Several countries use complex sampling designs where some groups or regions of the population have higher probabilities of selection. The main purpose of the design weights is to correct for the fact that in some countries respondents have different probabilities to be part of the sample due to the sampling design used. Applying the weights allows to correct for this and obtain estimates that are not affected by a possible sample selection bias. The design weights are computed as the inverse of the inclusion probabilities and then scaled such that their sum equals the net sample size.

• Post-stratification Weights: Design weights account for differences in inclusion probabilities and thus correct for bias that is introduced by the sampling design. However, other errors sources remain, including sampling error (related to attempting to measure only a fraction of the population) and non-response error (which may lead to a systematic overor under-representation of people with certain characteristics). Post-stratification weights are a more sophisticated weighting strategy that uses auxiliary information to reduce the sampling error and potential non-response bias. They have been constructed using information on age-group, gender, education, and region. The post-stratification weights are obtained by adjusting the design weights in such a way that they will replicate the distribution of the cross classification of age-group, gender, and education in the population and the marginal distribution for region in the population.

• Population Size Weights: Population size weights are used when examining data for two or more countries combined. The population size weights are the same for all persons within a country but differ across countries. These weights correct for the fact that most countries taking part in the ESS have different population sizes but similar sample sizes. Without this weight, any figures combining data from two or more countries might be biased, 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

A more detailed explanation of each type of weight and recommendations on how they should be utilized in analysis are provided in the "Weighting European Social Survey Data" document that can be found in the 'Documentation' tab.