Statistics for Environmental Engineers

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b23 = 0.0004 ± 0.0224    b123 = -0.0115 ± 0.0224

Method A gives results that are from 0.025 to 0.060 mg / L lower than Method B (on the log-transformed scale). This is indicated by the coefficient b2 = 0.0478 ± 0.0224. The difference between A and B on the log scale is a percentage on the original measurement scale.10 Method A gives results that are 2.5 to 6% lower than Method B.

If a 5% difference between methods is not important in the context of a particular investigation, and if Method B offers substantial advantages in terms of cost, speed, convenience, simplicity, etc., one might decide to adopt Method B although it is not truly equivalent to Method A. This highlights the difference between “statistically significant” and “practically important.” The statistical problem was important to learn whether A and B were different and, if so, by how much and in which direction. The practical problem was to decide whether a real difference could be tolerated in the application at hand.

Using PMA as a preservative caused no measurable effect or interference. This is indicated by the confidence interval [-0.004, 0.041] for b3, which includes zero. This does not mean that wastewater specimens could be held without preservation. It was already known that preservation was needed, but it was not known how PMA would affect Method B. This important result meant that the analyst could do nitrate measurements twice a week instead of daily and holding wastewater over the weekend was possible. This led to economies of scale in processing.

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