The focus of this Dagstuhl seminar, held in Dagstuhl Castle in November 2015, was to discuss experiences and methodological considerations when using crowdsourcing platforms to run human-centered experiments to test the effectiveness of visual representations. The inspiring Dagstuhl atmosphere fostered discussions and brought together researchers from different research directions. The papers provide information on crowdsourcing technology and experimental methodologies, comparisons between crowdsourcing and lab experiments, the use of crowdsourcing for visualisation, psychology, QoE and HCI empirical studies, and finally the nature of crowdworkers and their work, their motivation and demographic background, as well as the relationships among people forming the crowdsourcing community.