In this section we discuss how the method described above can be used to easily recruit participants. When we started doing experiments in Mannheim we sent a short email to all students of the university, describing the purpose of the experimental laboratory, that experiments can be entertaining and that participants will be rewarded. We invited them to fill in a form on the www where they could leave their email address and become members of our database. When they become members we also ask for their faculty, gender and age.
When we run experiments, we can selectively send emails to the members of our database and invite them to book an experiment. A MySQL-database allows participants to find out, at which dates how many places for an experiment are still open, and to book this date through the internet. The shop-up rate is in any case better than 80%, i.e. as good as anything else that one could achieve with other forms of recruiting.
A disadvantage of this approach is that the subject pool is biased. Well, possibly every subject pool is biased: If you recruit in front of the university library you select people who are perhaps more interested in books than other people. Our subjects are possibly more familiar with the internet than other subjects. Hence, we have about 70% male subjects in the database. Still, if necessary, we can selectively advertise our experiments, since we know the gender of most subjects.
Other applications of this method that we have used in Mannheim is submission of working papers and submission of contributions for a conference (used for the ESA 98 and the SPUDM 99 conference in Mannheim).