National indices are weighted when combined according to national population size estimates (from European Red List of Birds, BirdLife International 2015), but the quality of population estimates is very different between countries. Does it mean that weights are based on incorrect values?
Information on population size published by BirdLife International 2015 is the best source of information in Europe that is available. The data have been used in several studies, published in high profile scientific journals (e.g. Donald et al., 2007). There are indeed differences in the quality of information between countries, but the population estimates are robust and to obtain extra guarantees we asked the national coordinators to judge these estimates.