Statistics

Question 1

Explain PMSD in simple terms.

Answer 1

Percent Minimum Significant Difference = this represents the smallest significant value of the relative difference (percent effect) between the control mean and treatment mean.

Question 2

Does the lower PMSD bound benefit dischargers or regulators?

Answer 2

Dischargers.

Question 3

What happens if a test’s PMSD is above the upper bound, and the test showed a statistically significant difference between control and treatment means?

Answer 3

The test result is a “fail”, because even with the high variability, it still detected a toxic effect.

Question 4

What happens if a test’s PMSD is above the upper bound, and the test showed no statistically significant difference between control and treatment means?

Answer 4

The test must be repeated with a new effluent sample.

Question 5

How is MSD affected by increasing replicates and increasing within-test variation?

Answer 5

Increasing n leads to lower MSD.

Increasing within-test variation leads to higher MSD.

Question 6

Explain MSD in simple terms.

Answer 6

Minimum Significant Difference = the smallest difference between control mean and treatment mean that is statistically significant.

Question 7

What happens if a test’s PMSD is below the lower bound, and the test showed no statistically significant difference between control and treatment means?

Answer 7

The test result is a “pass”, because even with the unusually low variability, no toxicity was detected.

Question 8

Explain the difference between repeatability and reproducibility.

Answer 8

Repeatability measures the variation in measurements taken by a single instrument or person under the same conditions.

Reproducibility measures whether an entire study or experiment can be reproduced in its entirety.

Question 9

Explain how the PMSD upper and lower bounds were established.

Answer 9

1. EPA examined reference toxicant data from a large number of tests and laboratories.
2. They calculated the PMSD for each test, and calculated percentile figures for each method type.
3. The 10th and 90th percentile PMSD were selected as the upper and lower bounds for each method type.

Question 10

Does the upper PMSD bound benefit dischargers or regulators?

Answer 10

Regulators.

Question 11

What happens if a test’s PMSD is below the lower bound, and the test showed a statistically significant difference between control and treatment means?

Answer 11

The test is treated as if it found no significant difference, and the test result is a “pass”.