Instead of national population size estimates, other weights may be used in the calculation of supra-national population indices per species, e.g. area of habitat or precision of trend estimates. Why does PECBMS only apply population size estimates?
Weighting by population size is the most direct way to produce supra-national indices of bird populations. Weighting by area of suitable habitat is an alternative. However, this approach would imply that we ignore any differences in the population density of species between countries. This is inadequate because a consequence may be that a large country with low density of a species dominates the European trend. In our weighting procedure, a country hosting a large part of the European population may dominate the European trend, which is adequate.
Weighting by precision is obviously inadequate, as it would imply that very precise estimates from e.g. a small country may dominate the European trend.