1. Microworker. http://www.microworker.com. [Online; accessed 31-Mar-2015].
2. Zooniverse. http://www.zooniverse.org. [Online; accessed 31-Mar-2015].
3. A. Aker, M. El-Haj, M.-D. Albakour, U. Kruschwitz, et al. Assessing crowdsourcing quality through objective tasks. In LREC, pages 1456--1461. Citeseer, 2012.
4. V. Ambati, S. Vogel, and J. Carbonell. Collaborative workow for crowdsourcing translation. In Proceedings of the ACM 2012 conference on Computer Supported Cooperative Work, pages 1191--1194. ACM, 2012.
5. D. J. Campbell. Task complexity: A review and analysis. Academy of management review, 13(1):40--52, 1988.