Land's Formula MCL Confidence Interval

Land's Formula Confidence Interval (calculation from background)

Description:

The Land's Formula Confidence Interval MCL Test provides a method to compare the concentrations of parameters in a well to a comparison level such as a Maximum Contaminant Level (MCL) or Alternate Contaminant Level (ACL).

This method is presented in the USEPA document "Statistical Training Course for Ground Water Monitoring Data Analysis." The training course document is included as part of the USEPA 1992 statistical analysis guidance document.

Use:

The Confidence interval is useful for intra-well comparisons. No background monitoring location is required. A comparison level such as an ACL/MCL must be provided for the parameter. This procedure requires that data be natural log-transformed.

Remarks:

For a downward test, statistical significance is indicated if the LCL exceeds the comparison level.

For an upward test, statistical significance is indicated if the UCL exceeds the comparison level.

For a two-tailed test, statistical significance is indicated if either the UCL or LCL exceed the comparison level.

This method is presented in the USEPA 1992 Statistical Training Course Outline included with the 1992 guidance document. The 1992 guidance also states that confidence intervals should not be used to compare to an MCL because it is possible for samples to exceed the MCL, and yet the mean of confidence limit would still fall below the MCL, indicating no contamination, when in fact, regulatory limits had been exceeded. The 1992 guidance also points out that unlike tolerance intervals, confidence intervals do not indicate how often individual samples will exceed the MCL.