In 2005, WWF authors identified that the population data was potentially unrepresentative. As of 2009, the database was found to contain too much bird data and gaps in the population coverage of tropical species, although it showed "little evidence of bias toward threatened species". The 2016 report was criticized by a professor at Duke University for over-representing western Europe, where more data were available. Talking to "National Geographic", he criticised the attempt to combine data from different regions and ecosystems into a single figure, arguing that such reports are likely motivated by a desire to grab attention and raise money.