The "World Christian Encyclopedia" (WCE), and its successor The World Christian Database (WCD), is an authority on membership data for religions in the world, and its decades-long study by David Barrett and co-workers is a basis for many other estimates of Baháʼís in the world, such as adherents.com and ARDA. The data were released in editions of 1982, 2001, and 2018, and includes a break down by country. The WCE data has consistently reported higher numbers of Baháʼís than the reports of Baháʼí institutions. Danish researcher Margit Warburg studied Baháʼí membership data and feels that the WCE data is overstated for Baháʼís. For instance, WCE reports an estimated 1,600 Baháʼís in Denmark in 1995 and 682,000 Baháʼís in the USA. The number of registered Baháʼís at the same time were 240 and 130,000, respectively.