The recent distribution of some 28 million copies in the United States of the 2005 documentary Obsession has stirred heated debate about its contents. One lightening rod for criticism concerns my on-screen statement that "10 to 15 percent of Muslims worldwide support militant Islam."

"Obsession: Radical Islam's War Against the West" (2005)

Actually, Pipes did provide answers. He collected and published many numbers at "How Many Islamists?" a weblog entry initiated in May 2005.

First, though, an explanation of what I meant by Muslims who "support militant Islam": these are Islamists, individuals who seek a totalistic, worldwide application of Islamic law, the Shari‘a. In particular, they seek to build an Islamic state in Turkey, replace Israel with an Islamic state and the U.S. constitution with the Koran.

As with any attitudinal estimate, however, several factors impede approximating the percentage of Islamists.

• How much fervor: Gallup polled over 50,000 Muslims across 10 countries and found that, if one defines radicals as those who deemed the 9/11 attacks "completely justified," their number constitutes about 7 percent of the total population. But if one includes Muslims who considered the attacks "largely justified," their ranks jump to 13.5 percent. Adding those who deemed the attacks "somewhat justified" boosts the number of radicals to 36.6 percent. Which figure should one adopt?

• Gage voter intentions: Elections measure Islamist sentiment untidily, for Islamist parties erratically win support from non-Islamists. Thus, Turkey's Justice and Development Party (AKP) won 47 percent in 2007 elections, 34 percent of the vote in 2002 elections, and its precursor, the Virtue Party, won just 15 percent in 1999. The Islamic Movement's northern faction won 75 percent of the vote in the Israeli Arab city of Umm el-Fahm 2003 elections while Hamas, the Palestinian terrorist organization, won 44 percent of the vote in the Palestinian Authority in 2006. Which number does one select?

• What to measure: Many polls measure attitudes other than application of Islamic law. Gallup looks at support for 9/11. The Pew Global Attitudes Project assesses support for suicide bombing. Nawaf Obaid, a Saudi security specialist, focuses on pro-Osama bin Laden views. Germany's domestic security agency, the Verfassungsschutz, counts membership in Islamist organizations. Margaret Nydell of Georgetown University calculates "Islamists who resort to violence."

• Inexplicably varying results: A University of Jordan survey revealed that large majorities of Jordanians, Palestinians, and Egyptians wish the Shari‘a to be the only source of Islamic law – but only one-third of Syrians. Indonesian survey and election results led R. William Liddle and Saiful Mujani in 2003 to conclude that the number of Islamists "is no more than 15 percent of the total Indonesian Muslim population." In contrast, a 2008 survey of 8,000 Indonesian Muslims by Roy Morgan Research found 40 percent of Indonesians favoring hadd criminal punishments (such as cutting the hands of thieves) and 52 per cent favoring some form of Islamic legal code.

The Islamic Supreme Council of America's Hisham Kabbani says 5-10 percent of American Muslims are extremists.

These ambiguous and contradictory percentages lead to no clear, specific count of Islamists. Out of a quantitative mish-mash, I suggested just three days after 9/11 that some 10-15 percent of Muslims are determined Islamists. Subsequent evidence generally confirmed that estimate and suggested, if anything, that the actual numbers might be higher.

Negatively, 10-15 percent suggests that Islamists number about 150 million out of a billion plus Muslims – more than all the fascists and communists who ever lived. Positively, it implies that most Muslims can be swayed against Islamist totalitarianism.