Analysis of external quality assessment results in three dimensions

During the last six years there has been a marked change in the nature of interlaboratory variance in enzyme determinations in Finland. With ordinary two-dimensional plotting methods the change is difficult to see, because enzyme activities are different for each control serum. The decrease in the coefficient of variation (CV) is best seen when the CV is displayed as a trend surface as a function of time and enzyme activity. While the overall variance has decreased, the shape of the curve relating CV to enzyme activity has also changed. The dependence of CV on the enzyme activity level has decreased slightly. The use of trend surfaces to describe three-dimensional functions in external quality assessment is described.