According to the journal cited below, methodology should be investigated whenever zone diameters are consistently not within the mean range because there might be an error. Control charts such as Shewhart Diagram, and Westgard rules were mentioned to be helpful in interpreting changes and monitoring the performance, aside from those two, what are the other useful methods that we can utilize to assess the zone diameter issues?
King, A., & Brown, D. F. J. (2001). Quality assurance of antimicrobial susceptibility testing by disc diffusion. Journal of Antimicrobial Chemotherapy, 48(suppl_1), 71–76. https://doi.org/10.1093/jac/48.suppl_1.71