Background:

Sufficient sample size and minimal sample bias are core requirements in empirical data analyses. Combining opportunistic recruitment with an online survey and data collection platform yields new benefits compared to traditional recruitment approaches.

Objective:

The objective of this paper is to report on the success of different recruitment methods to obtain participants’ characteristics, participation behavior, recruitment rates, and representativeness of the sample.

Methods:

A longitudinal online survey was implemented as part of the European PASTA project, which was online between November 2014 and December 2016. During this period participants in seven European cities were recruited on a rolling basis. For all cities to reach a sufficient number of adult participants a standardized guide on recruitment strategy was developed. In order to make use of the strengths and to minimize weakness a combination of different opportunistic recruitment methods was applied. In addition, the city of Oerebro applied random sampling approach. In order to reduce attrition rate and improve real-time monitoring the online platform featured a participant and a researchers` user interface and dashboard.

Results:

A total of 10,691 participants were recruited. Most people found out about the survey through their workplace or employer (21.5 %), outreach promotion (20.8 %), and social media (17.4 %). The average number of questionnaires filled-in per participant varied between the cities, with the highest number in Zurich (11.0 ± 0.33) and the lowest in Oerebro (4.8 ± 0.17). Collaboration with local organizations, the use of Facebook and mailing lists, and direct street recruitment were the most effective approaches in reaching a high share of participants (p = <.001). Under consideration of invested working hours Facebook (p = <.001) was one of the most time-efficient methods. Compared to the cities census data, the composition of study participants was broadly representative in terms of gender distribution, however included younger and better educated participants.

Conclusions:

We observed that offering a mixed recruitment approach was very effective in achieving a high participation rate. The highest attrition rate and the lowest average number of questionnaires filled-in per participant were observed in Oerebro, who also recruited participants through random sampling. The findings suggest that people that are more interested in the topic are more willing to participate and to stay in a survey than those who are selected randomly and may not have a strong connection to the research topic. Whereas direct face-to-face contacts were very effective with respect to the number of recruited participants; recruiting people through social media was not only effective, but also very time-efficient. The collected data is based on one of the largest recruited longitudinal samples with a common recruitment strategy in different European cities.