Asynchronous fieldwork in cross-country surveys: an application to physical activity

Multi-country surveys often aim at cross-country comparisons. A common quality standard is conducting these surveys within a common fieldwork period, across all participating countries. However, the rate the target sample is achieved within that fieldwork period in each country varies substantially. Thus, the distribution of the interview month often varies substantially in the final sample. This may lead to biased estimates of cross-country differences if the variable of interest exhibit a non-constant trend over time. We demonstrate the implications of such an asynchronous fieldwork, using physical activity measured in the European Social Survey Round 7 collected between September 2014 and January 2015. Accounting for fieldwork month, we present a set of different post-estimation predictions. Physical activity varies across interview month, with countries with more observations during autumn were upward-biased, compared to countries with more observations during winter. Our results demonstrate how comparisons between countries are affected when interview month is omitted, and how accounting for interview month in the analysis is an easy way to mitigate this problem.