MULTIVARIATE CONTROL CHARTS - CONTROL CHARTS FOR CALIBRATION CURVES

The proposed multivariate control charts for p-dimensional vectors are an extension of the conventional control charts for one variable. The controlling quantity is the Mahalanobis distance of vector x from the central value vector x..: D = (x - x..)T. C-1. (x - x..), where C is the covariance matrix estimate. The quantity D has Hotelling's T2 dist ribution. A PC program was set up for the automatic graphical construc tion of such charts. The program draws the sequential chart of the qua ntity D as well as the position of the vectors x in the p dimensional control ellipsoid in the axes of the principal components. In this way a control chart was developed for the calibration curve in the photom etric determination of Fe3+ with sulfosalicyclic acid. Vector x was fo rmed by absorbance values for the calibration curve points (p = 5). Th e chart can assist in detection of even small disturbances of the cali bration curve.