Clarification from Pew

In response to my snarky request that Pew clarify its ways and means of counting Muslims, Brian Grim, Senior Researcher at the Pew Forum on Religion & Public Life, has sent along the following:

The apparent difference between the Pew Research Center estimates is explained by the fact that both the 2007 Muslim American Survey and the 2007 Religious Landscape Survey deal only with adults, while the more recent report, Mapping the Global Muslim Population, deals with the total population (i.e., both adults and children).  Based on questions in the Muslim American survey about household size and the religious affiliation of children in the household, we estimate that Muslims account for a larger proportion of the population that is under 18 than they do of the 18-and-over population; thus, our estimate of Muslims' share of the overall population is larger than our estimate of Muslims' share of the adult population.

The Pew Research Center's 2007 Muslim Americans survey (Muslim Americans: Middle Class and Mostly Mainstream) estimated that 0.6% of U.S. adults are Muslims.  The first chapter of the accompanying survey report, "How Many Muslims Are There in the United States," provides detailed information on how the 0.6% estimate was obtained and why we think it is superior to estimates derived from most surveys that are conducted only in English (or English and Spanish, as was the case with the 2007 Religious Landscape Survey, which found 0.4%).  (See U.S. Religious Landscape Survey Report II transcript at http://www.pewforum.org/events/?EventID=190.)

I'd only add that it would have been better had Pew given the actual .4 percent number in its Landscape Survey report, and explained why it believes the actual percentage of Muslims in is higher, rather than simply substituting what it believes to be the more accurate number. Results are results. But anyway, thanks, Brian.