Index > 3 Characterisation of radioactively contaminated sites >

3.10.5 Example of statistical data processing: Small areas of elevated activity

As discussed previously, the development of a DCGL starts with the assumption of a relatively uniform distribution of contamination. Some variability in the measurements is expected. This is primarily due to a random spatial distribution of contamination and uncertainties in the measurement process. The arithmetic mean of the measurements taken from such a distribution would represent the parameter of interest for demonstrating compliance.

Whether or not the radionuclide of concern is present in background determines the form of the statistical test. The Wilcoxon Rank Sum (WRS) test is recommended for comparisons of survey unit radionuclide concentrations with background. When the radionuclide of concern is not present in background, the Sign test is recommended. Instructions on performing these tests are provided in Section 3.10.3 and Section 3.10.4.

The Wilcoxon Rank Sum and Sign tests are designed to determine whether or not the level of residual activity uniformly distributed throughout the survey unit does not exceed the DCGLW. Since these methods are based on ranks, the results are generally expressed in terms of the median of the data set. When the underlying measurement distribution is symmetric, the mean is equal to the median. When the underlying distribution is not symmetric, these tests are still true tests of the median but only approximate tests of the mean. However, numerous studies show that this is a fairly good approximation. The assumption of symmetry is less restrictive than that of normality because the normal distribution is itself symmetric. If, however, the measurement distribution is skewed to the right, the average will generally be greater than the median. In severe cases, the average may exceed the DCGLW while the median does not. For this reason, EURSSEM recommends comparing the arithmetic mean of the survey unit data to the DCGLW as a first step in the interpretation of the data (see Section 3.10.8.4).

The Wilcoxon Rank Sum test is a two-sample test that compares the distribution of a set of measurements in a survey unit to that of a set of measurements in a reference area. The test is performed by first adding the value of the DCGLW to each measurement in the reference area. The combined set of survey unit data and adjusted reference area data are listed, or ranked, in increasing numerical order. If the ranks of the adjusted reference site measurements are significantly higher than the ranks of the survey unit measurements, the survey unit demonstrates compliance with the release criterion.

The Sign test is a one-sample test that compares the distribution of a set of measurements in a survey unit to a fixed value, namely the DCGLW. First, the value for each measurement in the survey unit is subtracted from the DCGLW. The resulting distribution is tested to determine if the centre of the distribution is greater than zero. If the adjusted distribution is significantly greater than zero, the survey unit demonstrates compliance with the release criterion.

Guidance on performing the statistical tests and presenting graphical representations of the data is provided in Appendix E.