EVALUATION OF STATE CENTRAL MILK LABORATORIES BY STATISTICAL ANALYSES OF STANDARD PLATE COUNTS

Abstract

Standard plate counts reported by analysts in 54 state central laboratories for "normal" milk samples sent out in 1961, 1962, and 1963 revealed that an incubation temperature of 32 C produced more colonies than 35 C, and that the overall variance, g2, of counts for any sample should not greatly exceed 0.012. Based on this variance, s2 = 0.012, 80% of the analysts should agree within 1.3 standard deviations of the logarithmic mean of 75% of the normal samples. A method for detecting outliers and judging the performance of individual analysts is described.