A study has found that data collected over a period of 53 years on elephant populations in Africa show that a dramatic decline in elephant populations has been recorded over the period. This study concludes that habitat loss and poaching are the reason for the dramatic decline in African elephants. However, the study, "Survey based inference of continental African elephant decline," published in the Proceedings of the National Academy of Sciences, found that it was difficult to measure the numbers for the entire continent and to monitor changes accurately. According to the study, forest elephant populations declined by an average of 90% from 1964 to 2016, while savannah elephant populations declined by 70%. In summary, this results in an average decline of 77%. The study used survey data from 475 sites in 37 countries, including Namibia, making it the most comprehensive assessment of African elephants to date. The declines were not evenly distributed: some populations disappeared completely, while others grew rapidly. Success stories of stable or growing populations could contribute to better conservation of the animals.