In response to our study,¹ Kivimäki et al suggested that reported sickness absence frequencies were underestimates of the total sickness absence burden in European Union (EU) member countries.² This concern about the veracity of these estimates led Kivimäki et al to caution policy makers to not use this data to inform policy. While we agree that more research is needed to establish potential biases associated with different approaches to ascertain accurate sickness absence data, we consider the European Survey on Working Conditions (ESWC) to be useful to inform the cross-national policy debate. Country specific studies contribute knowledge to the evidence base, but cross-national studies such as ours help to provide a stronger basis on which to make cross-national inferences. Furthermore, cross-national studies become more relevant as data accumulate and the data collection quality improves. We hope that Kivimäki and colleagues are not suggesting the ESWC be discontinued.

We consider the studies by Kivimäki et al to be some of the most relevant epidemiological studies of sickness absence predictors.^3–5 Although informative, these studies raise several issues in the context of cross-national comparisons. First, epidemiological cohorts in Finland and the United Kingdom represent very homogeneous and specific working populations (that is, municipal employees, hospital workers, and civil servants) with unknown generalisability to the national representative surveys studied in our paper or the ones referenced by Kivimäki and colleagues.^6,7 Second, a fundamental advantage of national workforce surveys is the ability to capture all workers, whereas registries may lead to an under-representation of marginal work groups typically not included in national registries. Indeed, Kivimäki et al are not arguing that the Finnish and British cohorts are representative of the countries’ workforces. Even so, labour market inequalities may cause temporary and less protected workers to be under-represented in the type of well designed cohort studies they have referenced.⁸ Temporary and less protected workers are important in the EU economy, and lack of knowledge about their labour market experiences as related to sickness absence could lead to their further marginalisation in the policy debate. Third, Kivimäki et al criticised the data collection method employed in the ESWC. We are not aware of any cross-national study comparing the reliability, validity, and performance of different sickness absence data collection methods. Concerns have been raised about who is placed on a sickness absence registry. Registered data are very conditioned by the country’s social security system criteria for sickness absence, which complicates between-countries comparisons.⁹ Therefore, whether registries are the gold standard in sickness absence studies remains a point of debate yet to be closed.

In addition, Kivimäki et al compared our results to two survey based studies from Finland and Britain, but differences in sample selection and questionnaire design between these studies may limit comparisons. Our study included people aged 15 years and older who had any paid job during the reference week, or who had a job but were temporarily absent. The recall period for sickness absence was 12 months. The Finnish study was based on employees aged 25–64 using a six month recall period for sickness absence.⁶ The British survey investigated the psychiatric morbidity prevalence among the British adult population. This study sampled workers aged 16–64 years and excluded workers with a psychosis diagnosis. Workers who were currently working or had been working in the last year were asked to report absence days due to their health or feelings.⁷ For these reasons, caution is needed if a direct comparison between these three studies is intended.

Finally, we agree with Kivimäki et al that potential bias in the ESWC could be present (see pp. 868–9 in our article). However, we would argue that the best sources of data to inform policy are derived from systematic efforts to collect sickness absence data in a clear and consistent fashion from a representative sample of the labour force within each country. We consider the evidence presented by Kivimäki et al to support our argument of the difficulty in establishing between-country comparisons due to the fragmented and insufficient sickness absence data available at the European Union level. We consider our results useful. Although the results are preliminary and may be subjected to scientific scrutiny, the comparative findings may provoke researchers to develop standards for sickness absence studies to facilitate between-country comparisons. In addition, we hope the observed differences will promote further investigation into root causes of between-country differences, especially between northern and southern EU members, as well as within-country gender differences. We certainly welcome cross-national collaborative efforts among the EU sickness absence researches to address all these issues.