Toledano MB, Smith RB, Brook JP, Douglass M, Elliott P (2015) How to Establish and Follow up a Large Prospective Cohort Study in the 21st Century - Lessons from UK COSMOS. PLoS ONE 10(7): e0131521. https://doi.org/10.1371/journal.pone.0131521

Large-scale prospective cohort studies are invaluable in epidemiology, but they are increasingly difficult and costly to establish and follow-up. More efficient methods for recruitment, data collection and follow-up are essential if such studies are to remain feasible with limited public and research funds. Here, we discuss how these challenges were addressed in the UK COSMOS cohort study where fixed budget and limited time frame necessitated new approaches to consent and recruitment between 2009-2012.

Web-based e-consent and data collection should be considered in large scale observational studies, as they offer a streamlined experience which benefits both participants and researchers and save costs. Commercial providers of register and marketing data, smartphones, apps, email, social media, and the internet offer innovative possibilities for identifying, recruiting and following up cohorts.

Using examples from UK COSMOS, this article sets out the dos and don’ts for today's cohort studies and provides a guide on how best to take advantage of new technologies and innovative methods to simplify logistics and minimise costs. Thus a more streamlined experience to the benefit of both research participants and researchers becomes achievable.

Heinävaara S, Tokola K, Kurttio P, Auvinen A. Validation of exposure assessment and assessment of recruitment methods for a prospective cohort study of mobile phone users (COSMOS) in Finland: a pilot study. Environ Health 2011;10:14.

The aim of the study was to evaluate the agreement between self-reported and operator-derived estimates of call time based on a three-month monitoring period, as well as the consistency of mobile phone use over time. Alternative approaches to improve participation in a cohort study of mobile phone users were also compared.

A total of 5,400 subjects were identified from network operators' subscriber databases for recruitment to the pilot study. Operator and questionnaire data were used to quantify mobile phone use. Operator data were available for a subset of the subjects for a three-month period in three consecutive years. We also evaluated the effect of the length of the questionnaire and one- or two-phase recruitment on participation.

The average response rate for both questionnaires and recruitment procedures was 12%. The response rate was not affected by the length of the questionnaire or the recruitment method.Operator data were available for 83% of the participants for 2007, the first study year. The agreement between self-reported and operator-derived call times decreased with the level of use among intermediate and heavy mobile phone users. During 2007-2009, mobile phone use increased fairly constantly over time.

The agreement between self-reported mobile phone use and operator databases was moderate and overestimation of the call time by participants was common. A prospective cohort study would be feasible in Finland, although the potentially low participation rate would increase the resources required for recruitment.


Schüz J, Elliott P, Auvinen A, Kromhout H, Poulsen AH, Johansen C, Olsen JH, Hillert L, Feychting M, Fremling K, Toledano M, Heinävaara S, Slottje P, Vermeulen R, Ahlbom A. An international prospective cohort study of mobile phone users and health (Cosmos): design considerations and enrolment. Cancer Epidemiol 2011;35:37-43.

There is continuing public and scientific interest in the possibility that exposure to radiofrequency (RF) electromagnetic fields (EMF) from mobile telephones or other wireless devices and applications might increase the risk of certain cancers or other diseases. The interest is amplified by the rapid world-wide penetration of such technologies. The evidence from epidemiological studies published to date have not been consistent and, in particular, further studies are required to identify whether longer term (well beyond 10 years) RF exposure might pose some health risk.

The "Cosmos" study described here is a large prospective cohort study of mobile telephone users (ongoing recruitment of 250,000 men and women aged 18+ years in five European countries - Denmark, Finland, Sweden, The Netherlands, UK) who will be followed up for 25+ years. Information on mobile telephone use is collected prospectively through questionnaires and objective traffic data from network operators. Associations with disease risks will be studied by linking cohort members to existing disease registries, while changes in symptoms such as headache and sleep quality and of general well-being are assessed by baseline and follow-up questionnaires.

A prospective cohort study conducted with appropriate diligence and a sufficient sample size, overcomes many of the shortcomings of previous studies. Its major advantages are exposure assessment prior to the diagnosis of disease, the prospective collection of objective exposure information, long-term follow-up of multiple health outcomes, and the flexibility to investigate future changes in technologies or new research questions.