Stage 4:  Execute calculations

The initial phase of the calculation is an extension of the detailed model design, in the form of a review of the decisions made concerning the input information, through testing elements of the calculation to gauge the sensitivity of the outputs to the precise choice of input value, and thus identify where the quality of input information may need to be revisited for further refinement.

Engineering methods can be implemented for many situations, but for large numbers of source and receiver locations and/or complex propagation paths dedicated software becomes practically essential. In particular, the use of such software enables the rapid exploration of different scenarios that may be relevant to the assessment. Further, such software often enables visualisations of the input data to be produced in order that a user can readily check the plausibility of the input data. However, the use of such software requires users to be fully aware of the manner in which it implements a particular procedure and in particular what assumptions are being made. Additionally, the documented descriptions of engineering methods are sometimes subject to a degree of interpretation as to the correct procedure, and this is a source of variation between different software packages. These considerations are such that proprietary modelling software should not be used in the absence of a working knowledge of the calculation routines.

At the outset of any calculation, the input data should be checked to confirm the plausibility of the results, particularly where the latter are within close proximity to a decision threshold or limit value. It is also essential to compare the predicted values with the simple calculated values estimated during the screening study, and to check the ranking of reported contributions attributable to each source against expectations.