Scientific assessment of the extent of "atheism" in various populations is beset with a number of problems. First in most of the world outside of East Asia the vast majority of the populations are believers in either a monotheistic or polytheistic system, typically being cited as 90% or more in countries like the United States or India. Consequently questions to assess non belief often take the form of any negation of the prevailing belief rather than an assertion of positive atheism and these will then be accounted, somewhat inaccurately, to rising "atheism".[3][4][5]
...
Studies and statistics
The demographics of atheism are substantially difficult to quantify. Different people interpret atheism and related terms differently, and it can be hard to draw boundaries between atheism, nonreligious beliefs, and nontheistic religious and spiritual beliefs. Furthermore, atheists may not report themselves as such, to prevent suffering from social stigma, discrimination, and persecution in some countries.[8]
Because some governments have strongly promoted atheism and others have strongly condemned it, atheism may be either over-reported or under-reported for different countries. There is a great deal of room for debate as to the accuracy of any method of estimation, as the opportunity for misreporting (intentionally or not) a category of people without an organizational structure is high. Also, many surveys on religious identification ask people to identify themselves as "agnostics" or "atheists", which is potentially confusing, since these terms are interpreted differently, with some identifying themselves as being agnostic atheists. Additionally, many of these surveys only gauge the number of irreligious people, not the number of actual atheists, or group the two together. For example, research indicates that the fastest growing religious status may be "no religion" in the United States, but this includes all kinds of atheists, agnostics, and theists.[9][10] Non-religious people make up 9.66%, while one fifth of them are atheists.[1]
Statistical problems
Statistics on atheism are often difficult to represent accurately for a variety of reasons. Atheism is a position compatible with other forms of identity. Some atheists also consider themselves Agnostic, Buddhist, Hindu, Jains, Taoist, or hold other related philosophical beliefs. Some, like Secular Jews and Shintoists, may indulge in some religious activities as a way of connecting with their culture, all the while being atheist. Therefore, given limited poll options, some may use other terms to describe their identity. Some politically motivated organizations that report or gather population statistics may, intentionally or unintentionally, misrepresent atheists. Survey designs may bias results due to the nature of elements such as the wording of questions and the available response options.
http://en.wikipedia.org/wiki/Demographics_of_atheism