Comparison of two methods

One of the most common tasks in the analytical laboratory is to determine whether or not two methods give comparable mean results within a predetermined confidence level. The test usually chosen for this task is the t-test. 

There are two different ways of carrying out this test. The first one involves taking a single sample and analysing it by both methods a number of times. The usual procedure is to undertake a number of analyses (preferably not less than 6) for the chosen sample with both methods and calculate the value of the t-statistic. This is then compared with the tabular value for the appropriate degrees of freedom at the selected confidence level. If the calculated value is less than the tabulated t value then the mean values, and hence the methods, are accounted equivalent. This method has the advantage that the number of replicates undertaken for each method does not have to be equal. However, it is not always recognised that for this test to be valid the precision of the two methods should be equal. The method used to compare the precisions of methods is the F-ratio test and is carried out as part of the procedure. 

The process may be illustrated with the following example. Table 21 gives some data from two methods, B and C, for estimating the silver content in photographic media. Method B is based upon an X-ray fluorescence procedure and Method C is a controlled potential coulometric method. 

As the tabular t value exceeds the calculated value, then we can conclude that the means are not significantly different at 95% confidence. 

However, there are occasions when variance equivalence is not observed. Another method, D, was used to determine the silver content of the photo- 

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Part 4 Performing a ranking test to determine if either analytical method or location affects the results as a systematic error (bias) and Part 5 Computing the efficient comparison of two methods as described by Youden and Steiner in reference [7],... 

Figure 34-2 Simple experimental design for Youden/Steiner comparison of two Methods (data shown in Table 34-2).

Table 34-2 Analytical data entry for comparison of two methods tests...

Collaborative Laboratory Studies Part 5 - Efficient Comparison of Two Methods... 

COMPUTATIONS FOR EFFICIENT COMPARISON OF TWO METHODS (COMP METH WORKSHEET)... 

This efficient statistical test requires the minimum data collection and analysis for the comparison of two methods. The experimental design for data collection has been shown graphically in Chapter 35 (Figure 35-2), with the numerical data for this test given in Table 38-1. Two methods are used to analyze two different samples, with approximately five replicate measurements per sample as shown graphically in the previously mentioned figure. 

O Sullivan, M., Gnemmi, E., Morris, D., Chieregatti, G., Simmonds, A., Simmons, M., Bridges, J., and Marks, V. (1979) Comparison of two methods of preparing enzyme—antibody conjugates Application of these conjugates for enzyme immunoassay. Anal. Biochem. 100, 100-108. 

Kolbl A, Leifeld J, Kogel-Knabner I (2005) A comparison of two methods for the isolation of free and occluded particulate organic matter. J Plant Nutr Soil Sci 168 660-667... 

Lumeij, J.T. and Scholten. 1989. A comparison of two methods to establish the prevalence of lead shot ingestion in mallards (Anas platyrhynchos) from the Netherlands. Jour. Wildl. Dis. 25 297-299. 

While the value for coefficient of variation is a general statement about the imprecision of a method, only the value for standard deviation can be used in any statistical comparison of two methods. The use of coefficient of variation assumes a constant relationship between standard deviation and the mean value and this is not always true (Table 1.5). 

In the following discussion, three types of air pollutant analytical data will be examined using principal component analysis and the K-Nearest Neighbor (KNN) procedure. A set of Interlaboratory comparison data from X-ray emission trace element analysis, data from a comparison of two methods for determining lead In gasoline, and results from gas chromatography/mass spectrometry analysis for volatile organic compounds In ambient air will be used as Illustrations. 

Figure 3. A comparison of two methods of producing starting conformations. With standard option -1, the conformations are generated from the preceding structure. With our -2 option, all conformations within a run are generated from the same, single starting point.

Grosjean, D E. C. Tuazon, and E. Fujita, Ambient Formic Acid in Southern California Air A Comparison of Two Methods, Fourier Transform Infrared Spectroscopy and Alkaline Trap-Liquid Chromatography with UV Detection, Environ. Sci. Technol., 24, 144-146 (1990). 

Explosive Performance, Comparison of Two Methods for Its Evaluation. Until, the middle of 1950 s, the choice of expls for blasting a particular rock was made on the basis of the following methods a) Trial and error gained from previous experience in similar rocks b) Calculation of meaningful performance parameters of expls from their chemical compns by means of laws of thermochemistry or thermodynamics or c) Laboratory determination of expl characteristics... 

S.J. Porter W.I. Duvall, Comparison of Two Methods for Evaluating Explosive Performance , Intnl Symp Mining Research, Rolla, Mo, Feb 1961... 

Expl performance, comparison of two methods for its evaluation 6 E424—E429... 

J Landry, S Delhaye, DG Jones. Determination of tryptophan in feedstuffs comparison of two methods of hydrolysis prior to HPLC analysis. J Sci Food Agric 58 439-441, 1992. 

Rock Breakage Using Explosives. See under Explosive Performance, Comparison of Two Methods for Its Evaluation in Vol 6, E424-R to E429-R... 

Ogawa, H., and Ogura, N. (1992). Comparison of two methods for measuring dissolved organic carbon in sea water. Nature 356, 696-698. 

Cusco I, Barcelo MJ, Biaget M, Tizzano EF. Implementation of SMA carrier testing in genetic laboratories comparison of two methods for quantifying the SMN1 gene. Hum Mutat 2002 20 452 159. 

G. W. Wheland, J. Chem. Phys. 2, 474 (1934). The Quantum Mechanics of Unsaturated and Aromatic Molecules A Comparison of Two Methods of Treatment. 

Comparison of Two Methods with Different Test Objects... 

Whitney, P. J. (1996). A comparison of two methods for testing defined formulations of PVC for resistance to fungal colonization with two methods for the assessment of their biodegradation. International Biodeterioration and Biodegradation, 37, 205-13. 

Davis, J.E., E.R. Stevens and D.C. Staff (1983). Potential Exposure of Apple Thinners to Azinphosmethyl and Comparison of Two Methods for Assessment of Hand Exposure, Bull. Environ. Contam. Toxicol., 31, 631-638. 

Lee, J. Reimaker, C. Wrolstad, R.E. 2009. Comparison of two methods for antho-cyanin quantification. Acta Hort. 810 831-834. 

Garuti, G., Meloni, S., Oddone, M. Neutron activation analysis of platinum group elements and gold in reference materials a comparison of two methods. J. Radioanal. Nucl. Chem. 245, 17-23 (2000)... 

Zelter, L., Regalado, M., Nichter, L. S., Barton, D., Jennings, S., and Pitt, L. Iontophoresis versus subcutaneous injection a comparison of two methods of local anesthesia delivery in children. Pain 44 73,1991. 

Hering J. G., Sunda W. G., Ferguson R. L., and Morel F. M. M. (1987) A field comparison of two methods for the determination of copper complexation bacterial bioassay and fixed-potential amperometry. Mar. Chem. 20, 299-312. 

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