EWCTS 2021 – Sampling
The sample used in the European Working Conditions Telephone Survey (EWCTS) 2021 is representative of those aged 16 years and over who are in employment and are resident in the country that is being surveyed. In 35 of the 36 countries, a single-stage, un-clustered sampling design was used. The kind of design employed is known as Random Digit Dialling (RDD), which implies that a computer randomly generates telephone numbers that match the configuration of telephone numbers in a country.
About the sampling procedure
The sampling frame used was mobile phones with numbers randomly generated with equal probability. These numbers were screened to remove those on ‘do not call’ lists, as well as inactive numbers or numbers for which it was clear the associated telephone was not in the country. This approach results in all sampling units having an equal probability of being selected with an estimated coverage of around 95% of the population.
In Sweden, the sample was drawn from the official register of resident persons, and was stratified by municipality, age and sex. Telephone numbers (which could be landline or mobile) were then added to this sample with a 99% match rate.
In each call, a number of screening questions were asked to confirm whether the respondent was eligible before beginning the interview.
Target sample sizes by country
For most EU countries, the target was 1,800 interviews. The exceptions were for the smallest and largest countries: Cyprus, Luxembourg and Malta (1,300 interviews); Poland and Spain (2,900), Italy (3,100), France (3,200) and Germany (4,100). Two countries also made use of the possibility to pay for a boosted sample which increased the national target, namely Belgium (3,000) and Slovenia (2,622).
Among non-EU countries, the six candidate and potential candidate (CPC) countries each had a target of 1,000 completed interviews, while the United Kingdom’s target was 2,100. Switzerland and Norway, which participated in the EWCTS at their own cost, had targets of 1,100 and 3,295 interviews respectively.
Across all countries, the total target was 68,817 and by the end of fieldwork the final number of interviews achieved after quality control was 71,758.
Detailed information on the sampling procedures can be found in the Sampling and weighting report.