|Cochran’s statistic C:
||where smax is the largest standard deviation amongst the si.
||Plot values for kij in the order of measurements made (by laboratory, e.g) and grouped by level. Draw in limits for 1% resp 5%.
||Plot values of the sample range, R (= xmax – xmin), against time. Action limits lie at k3sr. Warning limits lie at + k2sr
||for the new (n) resp. old (o) measurements.Compare F with Fmax for a certain number of number of degrees of freedom (= j + k – 2) and confidence level, which can be obtained from a table of overprocent points for the F-distribution (95%) resp. (99%).
||where UTL resp. LTL are the upper and lower tolerance limits, and PS is the process scatter, sp.