Why does the CLEA software generate different answers in forwards and backwards mode and which one is correct?
The CLEA software has two modes. The most commonly used is goalseek mode which generates assessment criteria using the Microsoft Excel goalseek function and is accessed from the Results worksheet. The second mode, called ratio mode, is for user entered soil concentrations, typically representative measured values from the site under investigation. Ratio mode can be switched on from the Basic Settings worksheet. Assessment criteria consistent with our framework reports are generated using the goalseek mode. Although it is possible to use ratio mode to find the soil concentration that equals an ADE / HCV ratio of one by manual trial and error, this will not always be consistent with the goalseek mode because of the different way in which background exposure is handled. This is discussed in both Environment Agency reports SR3 (Updated technical background to the CLEA model) and SR4 (CLEA Software (Version 1.04) Handbook) but a further explanation is given below. For threshold contaminants
