I see Norway tops the uncertainty list...31%-72%. Not very specific. But Vietnam is given as exactly 81%. Where do these numbers come from?
Source: Zuckerman, Phil. “Atheism: Contemporary Rates and Patterns”, chapter in The Cambridge Companion to Atheism, ed. by Michael Martin, Cambridge University Press: Cambridge, UK (2005).
You can find the study, as well as all its secondary bibliography and sources, here:
This type of international research is very complex, not least because the definition of atheism or lack of religion varies and because every country uses different methods to measure irreligion. Zuckerman himself:
Determining what percentage of a given society believes in God – or
doesn’t -- is fraught with methodological hurdles. First: low response rates; most
people do not respond to surveys, and response rates of lower than 50% cannot
be generalized to the wider society. Secondly: non-random samples. If the
sample is not randomly selected – i.e., every member of the given population has
an equal chance of being chosen -- it is non-generalizable. Third: adverse
political/cultural climates. In totalitarian countries where atheism is
governmentally promulgated and risks are present for citizens viewed as
disloyal, individuals will be reluctant to admit that they do believe in God.
Conversely, in societies where religion is enforced by the government and risks
are present for citizens viewed as non-believers, individuals will be reluctant to
admit that they don’t believe in Allah, regardless of whether anonymity is
“guaranteed.” Even in democratic societies without governmental coercion,
individuals often feel that it is necessary to say that are religious, simply because
such a response is socially desirable or culturally appropriate. For example, the
designation “atheist” is stigmatized in many societies; even when people directly
claim to not believe in God, they still eschew the self-designation of “atheist.”
Greeley (2003) found that 41% of Norwegians, 48% of the French, and 54% of
Czechs claimed to not believe in God, but only 10%, 19%, and 20% of those
respondents self-identified as “atheist,” respectively. A final methodological
problem: terminology. Definitions of specific words seldom translate well crossculturally.
Signifiers such as “religious” or “God” have different meanings in
different cultures (Beyer, 2003), making cross-national comparisons of beliefs
between markedly different societies tenuous. Despite the above methodological
limitations, we can make reliable estimates. Though methodological flaws persist,
in the words of Robert Putnam (2000:23): “we must make do with the imperfect
evidence that we can find, not merely lament its deficiencies.”
However, all studies to this effect yield similar results. Wikipedia lists studies by Gallup and the Dentsu Institute. While the questions asked and the methodology used varies, the same countries appear at the top: