Completion percentages of a task group for individual workers ("Turkers") on Amazon's Mechanical Turk, by percentile, compared to theoretical distributions. Completion rates break into three primary groups centered at 15%, 50%, and 95%. These groups manifest as three roughly linear segments when compared against a uniform or Gaussian distribution. A mixture of three Gaussians, with parameters determined by the E-M algorithm, results in a better fit.
Source: Stanford Visualization Group