In the UK, the Environmental Protection Act requires land to be determined as contaminated if it contains concentrations of substances such that there is significant risk of significant harm to certain defined receptors. A key step in assessing the possibility of significant harm is the comparison of measured soil concentrations of potentially harmful substances with appropriate assessment levels considered to represent tolerable levels of risk. Guidance on making the comparison using statistical techniques was issued in 2008 by the Chartered Institute of Environmental Health (CIEH) and Contaminated Land: Applications in Real Environments (CL:AIRE) (the CL:AIRE–CIEH method). When making decisions on the basis of a finite number of measurements from a very large amount of soil, there is always the possibility of error. It has become apparent that the CL:AIRE–CIEH approach does not necessarily control the likelihood of determining that land is not contaminated when in fact it is contaminated, and this is more likely when using the Chebyshev test. Although there are methods to control such likelihood, there is no clear basis for the necessary specifications. The difficulties can be obviated by making a determination of contamination at 50% probability, which is consistent with the statutory guidance. Alternatively, a different approach may be taken to decision-making.