January 12th, 2022

Sources of Variation. Homogeneity – the non-uniform distribution of microbes in the sample source (VSOURCE, where V = variability), the sample (VSAMPLE) and the specimen (VSPECIMEN) – contributes substantially to test result variability.

It’s Not the Method

A few years ago, a single set of fuel and fuel-associated water samples were used for two ASTM interlaboratory studies (ILS). You can read the details in the paper published in International Biodeterioration &…


December 21st, 2021

Most commonly, quantitative recovery applies when a method consistently detects a substantial percentage of the intended analyte in a specimen. Read on to learn more.

Analytes and Parameters

In chemistry, an analyte is a substance or material being measured by an analytical method. In microbiology, the analyte is either microbial cells or molecules. A parameter is a property used to quantify an analyte. Direct counting – using either a microscope or a flow cytometer – is…


October 19th, 2021

The tool you choose depends on the intended use.

Parts 1 Through 3 Recap
In Part 1 (July 2021), when I started this series on test method comparison, I provided an overview of several basic precision concepts:

Correlation coefficient
Regression curve

In Part 2 (August 2021) I explained why it is unrealistic to expect correlation coefficients between two methods, each based on a different parameter (e.g., CFU mL-1 and gene…


September 20th, 2021

The tool you choose depends on the intended use.



In August, I discussed the concept of attribute score agreement between two test parameters. Before continuing to the next part of my discussion, I’ll use a Venn diagram to further illustrate this concept. Figure 1 shows the respective data sets obtained by two test parameters – ATP-bioburden and culturable bacterial bioburden (bacterial CFU mL-1). The blue and red circles, respectively, represent the ATP and…


August 18th, 2021

The tool you choose depends on the intended use.

A Bit More About Relative Bias

In my last post I introduced the concept of relative bias. I wrote that unless there is a reference standard against which a measurement can be compared, only relative bias – the difference between test results obtained by different methods – can be assessed. In my example, I compared the results of two test methods for determining the concentration of end-use diluted metalworking fluids (MWF). …



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