It was the effect that the claim Amaral made in his book that Madeleine was dead had on his readers which concerned the McCanns. i.e. the fact that people would believe she was dead because of what he claimed in his book and stop looking for her.
Unfortunately it was not physically possible to quantify any such damage. And therefore IIRC it was deemed to be not proved.
It wouldn't have been difficult to engage a polling company in Portugal. Apparently in March 2008 the McCanns were "‘encouraged’ by poll support". Of course the polls also showed that people in the Algarve were unconvinced of their innocence before Amaral released his book. Another one after the release of Amaral's book might have helped, though, if the figures changed for the worst;
The Correio da Manha poll found that 38.6% of those questioned still believe Kate and Gerry McCann were involved in their daughter’s disappearance in the Algarve. On the other hand, 30.7% thought the couple, from Rothley, in Leicestershire, were completely innocent.
This compares to an earlier poll run by the Portuguese newspaper in which 39.9% of people interviewed thought the couple were guilty and 26.8% believed they were not to blame.
Read more:
http://metro.co.uk/2008/03/04/mccanns-encouraged-by-poll-support-23454/#ixzz4gUR9wVZU