Confidence interval 

Where z is the confidence value of interest, (e.g., 1.96 for 95% confidence).

ConfidenceInterval swf

Regarding confidence:

For regulatory purposes, the confidence interval probability is specified as 80% two tail (or 90% one tail).

  • Compare the upper confidence interval with the regulatory threshold.
  • Unless demonstrated otherwise, assume a “normal” distribution.

Statistically, you can expect some individual samples to fall below the threshold (non-hazardous) or exceed the threshold (hazardous). Since the 90% one tail confidence provides only a 10% chance that the true mean is above the upper confidence level, it is important to take a closer look at the outlier samples to understand why they do not fall within the expected range (expected based on the rest of the samples).

A closer look at the statistical data will helps to determine whether there is an adequate number of samples taken to represent the waste characteristics, where sampling points may be taken and whether the sample size was adequate. Click here for a statistical formulas and examples job aid.

Range where you would expect to find the true mean or true concentration of the chemical in the waste.