Replicate tests, determination

Whereas precision (Section 6.5) measures the reproducibility of data from replicate analyses, the accuracy (Section 6.4) of a test estimates how accurate the data are, that is, how close the data would represent probable true values or how accurate the analytical procedure is to giving results that may be close to true values. Precision and accuracy are both measured on one or more samples selected at random for analysis from a given batch of samples. The precision of analysis is usually determined by running duplicate or replicate tests on one of the samples in a given batch of samples. It is expressed statistically as standard deviation, relative standard deviation (RSD), coefficient of variance (CV), standard error of the mean (M), and relative percent difference (RPD). 

Figure 1 provides adjustments to critical values for CV p when a method is biased. The dotted curve gives critical values of CV-p as a function of bias for a statistical significance test performed at the 5% probability level. Because uniform replicate determinations of the bias were not made in the validation tests, the bias is treated as a known constant rather than an estimated value. The experimental design could be modified to permit determination of the imprecision in the bias by providing for uniform replication of the independent method as well as the method under evaluation. Then the decision chart could be modified to include allowance for variability of replicate bias determinations. 

We have presented a statistical experimental design and a protocol to use in evaluating laboratory data to determine whether the sampling and analytical method tested meets a defined accuracy criterion. The accuracy is defined relative to a single measurement from the test method rather than for a mean of several replicate test results. Accuracy here is the difference between the test result and the "true value, and thus, must combine the two sources of measurement error ... 

Based on previous testing of the research subject, the design of the full factorial experiment 23 with one replication to determine experimental error has been chosen. To eliminate the influence of systematic error in doing the experiment, the sequence of doing design point-trials, in accord with theory of design of experiments, has been completely random. The outcomes are given in Table 2.107. 

Figure 1. Dissolved oxygen vs time data. The experiments were basalt + synthetic Grande Ronde groundwater (B+SW) and synthetic Grande Ronde groundwater (SW) at 300 bars. Determination of uncertainties for B+SW data points is discussed in Table IV. Uncertainties for SW data were derived from replicable tests.

For each replicate vessel in the transformation test, these model parameters are to be estimated by regression analyses. The approach avoids possible problems of correlation between successive measurements of the same replicate. The mean values of the coefficients can be compared using standard analysis of variance if at least three replicate test vessel were used. The coefficient of determination, r, is estimated as a measure of the goodness of fit of the model. 

Table 11.2 shows the EML testing results of one laboratory for two different matrices. The EML Value listed in the tables is the mean of replicate EML determinations for each nuclide. The contractors results ( Reported Value ) are divided by the EML Value to obtain the Ratio. The column titled Last Evaluation gives the laboratory a comparison of its present and preceding performance. 

There are several reports using animal eells to test the MEIC ehemieals (Shrivastava et al., 1992 Rommertetal., 1994 Rougetetal., 1993). Shrivastava etal., used freshly isolated rat hepatoeyte eells and two transformed eell lines. Comparable results were obtained with the three eell types indieating that rat hepatoeytes are no more resistant to toxie ehemieals than are other eells. They determined viability of the eells with 1) morphologieal studies 2) lactic dehydrogenase activity in the hepatoeytes 3) the ability of the cells to take up the dye tryptan blue. They got much the same results with the three techniques. Rommert et al., simply determined the effect of the toxins on the ability of the cells to replicate. They determined cell numbers with a Coulter Counter after 48 hours. Rouget et al., followed the effect of the toxins by determining the ability of cells to reduce the tetrazolium dye MTT and the ability of cells to take up the dye neutral red. The authors maintained that the two methods provided comparable results. 

Taking into account formula (1) the value of grain modulus of elasticity E = 7,52 MPa (as the average value from all testing replications), was determined. 

An alternative approach for collaborative testing is to have each analyst perform several replicate determinations on a single, common sample. This approach generates a separate data set for each analyst, requiring a different statistical treatment to arrive at estimates for Grand and Csys-... 

Untreated (control) soil is collected to determine the presence of substances that may interfere with the measurement of target analytes. Control soil is also necessary for analytical recovery determinations made using laboratory-fortified samples. Thus, basic field study design divides the test area into one or more treated plots and an untreated control plot. Unlike the treated plots, the untreated control is typically not replicated but must be sufficiently large to provide soil for characterization, analytical method validation, and quality control. To prevent spray drift on to the control area and other potential forms of contamination, the control area is positioned > 15 m away and upwind of the treated plot, relative to prevailing wind patterns. 

Hepatitis C is diagnosed by testing for anti-HCV in the serum. The disease is confirmed by the presence of HCV RNA. HCV RNA levels quantify viral replication and are used to determine if antiviral treatment for HCV is effective. 

---