At this point, even the Warren Commission probably wouldn’t buy the National Jewish Population Survey’s numbers.

The United Jewish Communities-sponsored 2000-2001 NJPS, the largest and most expensive survey of American Jews yet, raised eyebrows when it found only 5.2 million Jews in the United States, a drop of 300,000 from the UJC’s 1990 tally.

Critics, most notably San Francisco demographer Gary Tobin, claimed the NJPS was systematically undercounting cultural Jews and Jews in the Western United States. Allegations of sloppiness joined charges of methodological flaws when it was revealed data concerning roughly 175,000 phone surveys had been lost.

And now, in the latest vote of no confidence for the massive NJPS survey, the 2006 American Jewish Yearbook, which came out Monday, Jan. 1, claims the U.S. Jewish population is roughly 6.4 million, based on a series of smaller surveys culled by University of Miami professor Ira Sheskin and University of Connecticut professor Arnold Dashefsky.

Smarting, the UJC is considering taking its ball and going home — and getting out of the survey business.

Though no final decision has been made regarding a 2010 population study, it “would be both very expensive and may not serve the needs of the federation in terms of moving forward,” said Barry Swartz, the UJC’s senior vice president of federation services.

And if the UJC does indeed cease its surveys, it won’t be a move Tobin regrets — actually, he’ll regret it didn’t happen 37 years ago.

“For the 1970 study, no report was ever issued and the data ended up essentially being lost. In 1980, the study failed to launch. In 1990 there was a huge controversy over the intermarriage rate, which in 2000 was rescinded as an incorrect number by the UCJ. They said it was 52 percent and, later on, they said ‘never mind.’ And of course 2000 was a disaster, with the big deal being made about the Jewish population declining so precipitously,” he said.

“Out of all the work that has been done at the national level from 1970 to 2000, the only two numbers anyone remembers are the 52 percent intermarriage rate and 5.2 million Jewish population — both of which are wrong.”

A smaller-scale survey Tobin conducted at the same time revealed an estimated 6.7 million American Jews. And while some demographers, including Sheskin, claimed Tobin was grasping in his definition of who is a Jew, the numbers he uncovered jibe with subsequent surveys.

There are several reasons why crunching the numbers is important, according to Jonathan Sarna, a professor of Jewish history at Brandeis University.

A recent study by Sergio DellaPergola of the Jewish People Planning Policy Institute in Jerusalem showed that Israel’s Jewish population had surpassed the American Jewish population — at least based on the NJPS estimate.

The fluidity of estimates about the American Jewish community in some ways has become a contest for “bragging rights” about which is the largest Jewish community in the world, Sarna said.

In fact, the press release sent out earlier this month by the American Jewish Committee, said its estimate of 6.4 million American Jews “indicates that American Jewry remains the largest Jewish community in the world.”

But Sarna says the biggest question raised by the varying studies is “How do you count Jews?”

There’s disagreement over whether such surveys should count self-identifying Jews, anyone who lives with a Jew or is somehow associated with Jews, or only Jews who are both self-identifying and who associate with some Jewish institution.

“You’re not going to get 20 demographers to agree about who is a Jew any easier than you’d get 20 rabbis,” said Tobin.

“Whether it’s 6 million or 6.2 million or 6.5 million Jews in the United States depends on how you count them and whom you’re counting. But most importantly, the Jewish population is not declining.”

If the UJC were to scrap the NJPS, it might conduct more targeted studies, perhaps looking at different aspects of philanthropy or taking a look at specific age groups such as the baby boomers, he said.

The UJC spent millions of dollars on the 2001 survey, and “if we were to follow the same exact regimen, that number would grow exponentially,” Swartz said. “We want to make sure we get value in return.”

Joe Eskenazi is a j. staff writer; Jacob Berkman is a JTA staff writer.