If necessary, the auditor may also require an explanation why the particular set of variables are being used in the model. Non-statistical cost models may not always have weight and complexity as independent variables. Size may be implied by amount of energy, thrust level, tank volume, or parts count. Complexity may be implied by temperature, efficiency, thermodynamic cycle type, or manufacturing processes and packaging density. In many cases, non-statistical cost models can be constructed by equating cost to these directly measurable parameters.
Care has to be taken to equate cost to independent technical parameters.
In general, non-statistical cost models are used at a top level, i.e., are often not divisible into the typical WBS labor and material cost components.