40 C.F.R. § 264 app IV to Part 264

Current through September 30, 2024
Appendix IV to Part 264 - Cochran's Approximation to the Behrens-Fisher Students' t-test

Using all the available background data (nb readings), calculate the background mean (Xb) and background variance (sb2). For the single monitoring well under investigation (nm reading), calculate the monitoring mean (Xm) and monitoring variance (sm2).

For any set of data (X1, X2, . . ., Xn) the mean is calculated by:

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and the variance is calculated by:

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where "n" denotes the number of observations in the set of data.

The t-test uses these data summary measures to calculate a t-statistic (t*) and a comparison t-statistic (tc). The t* value is compared to the tc value and a conclusion reached as to whether there has been a statistically significant change in any indicator parameter.

The t-statistic for all parameters except pH and similar monitoring parameters is:

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If the value of this t-statistic is negative then there is no significant difference between the monitoring data and background data. It should be noted that significantly small negative values may be indicative of a failure of the assumption made for test validity or errors have been made in collecting the background data.

The t-statistic (tc), against which t* will be compared, necessitates finding tb and tm from standard (one-tailed) tables where,

tb = t-tables with (nb-1) degrees of freedom, at the 0.05 level of significance.

tm = t-tables with (nm-1) degrees of freedom, at the 0.05 level of significance.

Finally, the special weightings Wb and Wm are defined as:

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and so the comparison t-statistic is:

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The t-statistic (t*) is now compared with the comparison t-statistic (tc) using the following decision-rule:

If t* is equal to or larger than tc, then conclude that there most likely has been a significant increase in this specific parameter.

If t* is less than tc, then conclude that most likely there has not been a change in this specific parameter.

The t-statistic for testing pH and similar monitoring parameters is constructed in the same manner as previously described except the negative sign (if any) is discarded and the caveat concerning the negative value is ignored. The standard (two-tailed) tables are used in the construction tc for pH and similar monitoring parameters.

If t* is equal to or larger than tc, then conclude that there most likely has been a significant increase (if the initial t* had been negative, this would imply a significant decrease). If t* is less than tc, then conclude that there most likely has been no change.

A further discussion of the test may be found in Statistical Methods (6th Edition, Section 4.14 ) by G. W. Snedecor and W. G. Cochran, or Principles and Procedures of Statistics (1st Edition, Section 5.8 ) by R. G. D. Steel and J. H. Torrie.

Standard T-Tables 0.05 Level of Significance

Degrees of freedom t-values (one-tail) t-values (two-tail)
16.31412.706
22.9204.303
32.3533.182
42.1322.776
52.0152.571
61.9432.447
71.8952.365
81.8602.306
91.8332.262
101.8122.228
111.7962.201
121.7822.179
131.7712.160
141.7612.145
151.7532.131
161.7462.120
171.7402.110
181.7342.101
191.7292.093
201.7252.086
211.7212.080
221.7172.074
231.7142.069
241.7112.064
251.7082.060
301.6972.042
401.6842.021

Adopted from Table III of "Statistical Tables for Biological, Agricultural, and Medical Research" (1947, R. A. Fisher and F. Yates).

40 C.F.R. 264 app IV to Part 264

47 FR 32367, July 26, 1982