What happens is that the model produces what is known as a "ready reckoner" which is given to the bidders.

BBC: Why all the West Coast bids were wrong

This ready reckoner tells each bidder the financial implications of their respective forecasts of how much they think they can increase revenues.

BBC: Why all the West Coast bids were wrong

One very interesting question is how much extra capital First Group and the other bidders would have been forced to hold if the department's model and ready reckoner had been correct.

BBC: Why all the West Coast bids were wrong

It was because of assumptions in the department's own model about what would happen to inflation and passenger numbers - which in turn meant that the ready reckoner given to bidders was wrong.

BBC: Why all the West Coast bids were wrong