The number of adherents of a religion spread across many countries is exceptionally difficult to estimate accurately. Few national Baháʼí communities have the administrative capacity for efficient enumeration of membership, and Baháʼí membership data does not break out active participation from the total number of people who have expressed their belief. Due to its small size, few censuses or religious surveys include the Baháʼí Faith as a separate category, and some government censuses count Baháʼís as Muslims or Hindus. Those that have included the Baháʼí Faith are known to under- or overestimate many proportionally small groups. Country-level detail from "World Christian Encyclopedia", on which many other estimates rely, tends to be much higher than the numbers available from Baháʼí institutions, which themselves are counting declared belief, not active participation. Analyzing data on activity levels, Danish sociologist Margit Warburg suggested that by 2001 registered Baháʼís were reliably over 5 million, and active participants were approximately 900,000 (18% of registered Baháʼís).