Demonstration of the effect of bias on a closure phase measurement. Plot a): the closure phase trace resulting from a binary pair consisting of a resolved primary of radius R_⋆ and an unresolved secondary with flux ratio r = 0.01 for separations (s = 10 R_⋆,100 R_⋆,1000 R_⋆). Plot b): the closure phase trace for the 100 R_⋆ system recovered by the three estimators. correctly returns the ideal closure phase, whilst and do not. Both plots show the single source closure phase signal (thin solid line) for reference. Plots c) and d) show the closure phase calculated using and respectively as in plot b) along with the statistical uncertainty on the measurement after 10³, 10⁴ and 10⁵ frames.