Faridani, Siamak, et al. “A Ground Truth Inference Model for Ordinal Crowd-Sourced Labels Using Hard Assignment Expectation Maximization”. Proceedings of the AAAI Conference on Human Computation and Crowdsourcing, vol. 1, no. 1, Nov. 2013, pp. 20-21, doi:10.1609/hcomp.v1i1.13133.