Sure.
What I've taken into account, using exactly the same data as Professor Lamb, is to look not only at what donors said they felt about tax credits; look also at how they actually behaved subsequently. After saying tax credits were important to them, did they intend to claim them? The answer, for a very significant number, was no. Another question was asked: Do you take tax credits into account in your charitable giving? The answer again was a very strong no.
So we look at both views and anticipated behaviours, and the numbers shrink progressively as you move through that analytical sequence.