Recruitment, response analyses and methods
Project goals
For many scientific questions, it is important that the composition of the study population under investigation corresponds approximately to the composition of the underlying population so that representative (generally valid) statements can be made on the basis of the study results. Unfortunately, the willingness to participate in health studies has fallen continuously in recent years, so that the representativeness of the study populations is less and less guaranteed. There is therefore a need for (1) procedures to improve the willingness to participate (response) and (2) methods for the unbiased estimation of relevant population parameters.
The expert group ‘Recruitment, response analyses and methods’ is therefore dedicated to the following tasks and objectives:
- Development of methods to design the invitation process in such a way that the willingness to participate in the baseline study and all follow-up studies is as high as possible and independent of the socio-demographic characteristics of the invited persons
- Analyses on the question of which population groups are more or less frequently represented in the German National Cohort (NAKO)
- Determination of correction weights for prevalence estimates that take into account the influence of the sampling design as well as the influence of the willingness to participate and the sociodemographic characteristics of the invited persons.
Speakers
Dr. Stefan Rach
Dr. Matthias Sand (Deputy)
Prof. Dr. Carsten Oliver Schmidt (Deputy)
Results
Participation: From 2014 to 2019, a total of more than 1.3 million people were invited to take part in the NAKO. 205,414 people (15.6%) accepted this invitation and completed the baseline examination. The acceptance rate for participation in the study was higher among women than men and increased with age in both genders. The response rate was highest in rural areas, lower in small towns and suburbs and lowest in large cities.
Representativeness: If one compares the NAKO study sample with the general population in the study regions, it is noticeable that the NAKO sample contains:
- People with a low and medium level of education are less frequently represented, while people with a high level of education are more frequently represented
- People with a non-German nationality or migration background are less frequently represented
- People from single households are less frequently represented, while people from households with 3 or more people are more frequently represented.
Study invitation: The study confirmed the observation of previous studies that the colour of the envelope of a study invitation has no significant influence on the probability of an active response or study participation.
Publications
- Rach, S., Sand, M., Reineke, A. et al. The baseline examinations of the German National Cohort (NAKO): recruitment protocol, response, and weighting. Eur J Epidemiol (2025). https://doi.org/10.1007/s10654-025-01219-8
- Langeheine M, Pohlabeln H, Ahrens W, Günther K, Rach S. Study invitations with envelopes made from recycled paper do not increase likelihood of active responses or study participation in the German National Cohort. BMC Res Notes. 2019;12(1):468. http://doi.org/10.1186/s13104-019-4510-y
- Rach S, Günther K, Hadeler B. Participants who were difficult to recruit at baseline are less likely to complete a follow-up questionnaire – results from the German National Cohort. BMC Med Res Methodol. 2020;20(1):187. http://doi.org/10.1186/s12874-020-01073-0