Dr. Kieran McQuinn:

That is the way it has been done in the past. Ideally, we want to be able to determine what we think is the sustainable level of taxes, then we look at what actually comes in and identify the difference between the two figures. The problem is estimating a sustainable rate because we are dealing with a type of black box. I mean we do not know the individual firms, their practices and there are confidentiality clauses etc. around that so all that makes identifying a figure very difficult. So we end up having to use relatively straightforward rules of thumb.