Satterthwaite Two-Sample T-Test
Satterthwaite Two-Sample T-Test
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Description:
The Satterthwaite Two Sample t-test is provided in the U.S. Navy 1999 Guidance Document as a test to determine if compliance location measurements are statistically elevated when compared to background. It is a parametric test, and therefore a normal distribution is required. U.S. Navy 1999 Guidance recommends replacing non-detects with the detection limit or one-half of the detection limit. In ChemStat, the test can be performed at the 99%, 95%, or 90% confidence levels.
Use:
Use the Satterthwaite test to determine if compliance observations as a group are statistically elevated when compared to background as a group. The test can not be used to determine statistically elevated levels at individual sampling points.
Remarks:
At least two background measurements and two compliance measurements are required to prevent divide by zero errors. But this is a bare minimum, and many more samples are recommended for effective use.
U.S. Navy (1999) also recommends an equal number of background and compliance measurements to simplify calculations when done by hand. However, when using ChemStat, this simplification is unimportant, so any number of background and compliance measurements are suitable.