As described in the previous column (1), often there is confusion in multidisciplinary uses of statistical methods for analytical
methods due to the variation in terminology, assumptions, and specific uses within different technical disciplines. Certain
analytical methods of analysis are concerned only with overall error of analysis or correlation; others emphasize repeatability
and reproducibility, and yet others report only bias and precision. Often, engineers look at total error analysis as tolerance
stacking so that the sums of all errors in a specific dimension are less than the total maximum allowable tolerance.
Table I: Sample data
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As noted in Part I of this column series, a unification of multiple disciplines into a reasonable set of statistical parameters
remains useful for a broad presentation of clinical analytical data. A single set of statistical parameters is useful to a
multidisciplinary team involved in looking at analytical method comparison.
Table II: Sample data in Excel format
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This column describes the use of an Excel spreadsheet that computes and displays the statistical parameters described in Parts
I and II of this series. For each parameter, the basic Excel computation nomenclature is described. The equations and terminology
in this column are consistent with Clinical and Laboratory Standards Institute (CLSI) guidelines (2); please refer specifically
to these guidelines referenced below for your own assessment. These statistical analyses evaluate the accuracy of a test method
compared with a reference method measuring the same analyte.
Table III: Excel spreadsheet showing all calculations
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Readers can refer to Part I of this column series for more detailed definitions of terms and to references 3–5 for additional
descriptions and worked problems associated with the individual statistics demonstrated in this article. Note the data used
for Parts I and II are shown in Tables I and II, and the entire Excel spreadsheet with computed results is shown in Table
III. Also note the specific locations of data and calculations by rows and columns when referring to the Excel computation
formulas.
Basic Definitions
A comparison of methods records differences between a test method and a comparative or reference method:
- X: comparative or reference method
- Y: test method
- xi: observation i from comparative (reference) method
- yi: observation i from test method
- n: total number of observations i
For clarity, this column assumes the comparative (reference) method is a traceable reference method that has better precision
than the test method, which can be achieved by averaging replicate reference measurements if necessary.
Measurement Error
The measurement error (ei) is the test method measurement minus the reference method
e = Test Measurement – Reference Method
or equivalently, using the CLSI definitions, the measurement error for the for the ith observation is
ei = yi – xi
