Financial models used in the PFI are split roughly evenly between ones which calculate a Unitary Charge automatically and ones which do not. The automatic models are programmed to find the smallest Unitary Charge which meets certain criteria that the bidder and its financial back find acceptable. The manual models simply calculate the consequences of a Unitary Charge specified by the bidder; it is for the analyst to inspect the results, determine whether criteria are met, and to type in different Unitary Charges until they are.
When building such models itself, Operis favours manual models; they are simpler, and therefore quicker to build, more reliable and easier to understand, and they leave the final pricing to a human analyst who can be questioned by his colleagues rather than to a black box which only some members of the bidding team understand.
There is a slightly more subjective element to determining a Unitary Charge with a manual model than with an automatic one. Given unchanging assumptions, an automatic model will invariably suggest the same Unitary Charge regardless of who is operating it. Two individuals may get slightly different results from a manual model depending on how they interpret the output.