Since its introduction in 2003, the PSAH has been adjusted several times to improve its cost-effectiveness: by targeting areas with high biodiversity benefits, areas with high risk of loss (to ensure additionality), and areas with low opportunity costs. Given that there were 1.8 million hectares enrolled in the programme, its conservation impact has been fairly low. This can be partly explained by the low weight given in the design of the programme to environmental criteria vis-a-vis social and administrative criteria (such as complementarities with other programmes) - environmental criteria represented 40% of the weight in 2006 and 29% in 2010.