One of the problems regarding data on festival participation is that many of such events are held in public spaces and we only have access to rough figures estimated by traffic and citizen safety services.
In the case of La Mercè, a few years ago the Institut de Cultura de Barcelona (ICUB, Culture Institute of Barcelona) began to implement more thorough systems for calculating citizen participation. The most recent innovation has been the installation of thermal sensors in Ciutadella park and Montjuïc castle that have provided more reliable data for places where it’s more difficult to calculate numbers of people attending. In the Informe de participació Mercè 2016 (Report on participation to La Mercè 2016) you’ll find more information on this topic.
We should add that implementing these techniques is very costly and event organisers cannot always afford them. As a result, rather than applying these solutions to other events, we should seek to define a set of indicators for popular culture that transcend numbers of participants.