Dr. Elaine Byrne:

It measures perception. There are eight different surveys of different groups of population in each country and those are correlated. There are very few convictions in the world for corruption so it would be very difficult to have a rank that measures actual corruption. By its nature corruption is clandestine. There are problems with measuring just perception, and Transparency International and other agencies have acknowledged that. However, there is also a consistency about these rankings over a period of time and correlated to other indices.