R E Thiers | PubMed Health

PubMed1976

Sample stability: a suggested definition and method of determination.

We propose defining the stability of any chemical constituent of stored samples in terms that are quantitatively related to the precision of the measurement by which it is determined. We suggest that a constituent may be considered stable, for a stated period and under exactly defined conditions, when the average change in its measured value is less than a chosen number, K, of standard deviations of the data obtained by the measuring method over the concentration range in question. Based on this definition a technique utilizing a graphical truncated normal sequential test is presented as the appropriate experimental and statistical design for measuring stability. The statistical basis for the proposal is presented.

Chemistry, Clinical↗

PubMed1975

Statistical evaluation of method-comparison data.

Comparison of two methods for determining a substance is a common procedure in clinical chemistry. Almost without exception, normal theory statistics are used with the tacit assumption that the data under test show gaussian distributions. Experience has led us to question this assumption for populations of differences between values obtained by two methods applied to common samples and for populations of paired values for each method as used in correlation analysis. Data are presented from our laboratory to illustrate these points, as well as an examination of data from relevant papers that appeared in this journal from 1968 through 1973. These examples indicate that incorrect interpretation of results because of insufficient attention to non-gaussian distribution is a real and present danger. We present a systematic approach to the statistical aspect of comparing methods that enables one to avoid this problem.

Blood Chemical Analysis↗