There isn’t a whole lot of ambiguity about 1 percent — which happens to be the affiliation rate for San Francisco’s young Jewish couples.

But hey, considering the 3 percent margin of error in the S.F.-based JCF’s recently completed Jewish Community Study, perhaps a whopping 4 percent of young Jewish couples belong to a synagogue.

Of course, that also means that the affiliation rate could be two below zero.

Bruce Phillips can only laugh at that. No, San Francisco doesn’t feature a negative affiliation rate (even if it feels that way at times).

The question, though, remains: Why aren’t young couples joining up?

“Probably because they’re having too much fun,” quipped Phillips, the demographer behind the federation’s study.

There are, however, other ways to measure the strength and vibrancy of a community, and, Phillips assures, San Francisco is doing all right there. And, while one of the major effects of his study has been the recognition of the South Peninsula as the new locus of Jewish activity in the Bay Area, San Francisco isn’t ready to give up its title as the traditional epicenter.

In an interview focusing on San Francisco, the Los Angeles-based demographer touched on many issues.

Perhaps most notably, he indicated, San Francisco is growing Jewishly. After two decades of stagnation, the city’s Jewish population has soared by nearly 20,000 since the federation’s 1986 survey.

And San Francisco’s Jewish population has grown even while its overall Caucasian population has plummeted. Phillips thinks he knows why.

“Jews tend to be urban and educated and tend to like cultural stuff. They like living in the city and taking advantage of it,” he said.

“Other whites are less likely to be attracted to those urban amenities and move out more, out of the metro area altogether, to places like Fresno and Visalia.”

San Francisco, for Jews and non-Jews alike, is the gateway to the Bay Area for outsiders. A typical situation would unfold with a young man or woman moving to the area, meeting his future spouse, living for a few wild and crazy years in San Francisco before settling down on the Peninsula to raise a family.

“It’s like when you live in Boston or Manhattan as a young couple. Even though they’ll most likely move down the Peninsula, [San Francisco] remains part of their psyche,” said Phillips.

The city is not all fun and games, however. Around one of every eight Jews in San Francisco is categorized as poor, making the city the second hardest-up in the region, a shot across the bow of the stereotype that San Franciscans are universally well-to-do.

Gays and lesbians were more apt to suffer from financial hardships than the Jewish population in general, as, Phillips pointed out, many are clustered in low-income professions such as teaching, social work and the arts. (The study found that 32 percent of the Bay Area’s Jewish homosexuals are poor.) And, while San Francisco is true to form as a heavily gay and lesbian city, it’s not the only place to be.

“The gay population, to a certain extent, like the Russian population, has moved out of San Francisco. People feel more welcome than they used to in other places,” he said.

San Francisco also features an eye-catching proportion of children being raised in single-parent families — 28 percent.

Phillips speculated that San Francisco’s single-parent families may opt to stay in rent-controlled apartments rather than move somewhere else.

Such tatistics have already spurred the federation to target single-parent households with social programs. And disturbing figures such as young couples’ low affiliation rate demonstrate to synagogues and other groups that they have much work to do.

“Synagogues tend to be family oriented. And that is a challenge for San Francisco,” said Phillips. “That is a group that really has to be reached out to.”

Other San Francisco facts of note:

• Thirty-eight percent of San Francisco’s Jews have moved here since 1990; 22 percent were born in the city.

• When asked where they expected to be living in three years, 73 percent of S.F. Jews said they expected to be in the same place, the second-highest percentage locally to Marin residents’ 82 percent.

• In San Francisco, 19 percent of Jewish households are singles younger than 40, and 26 percent are singles over 40. Both figures are, by a slim margin, the highest in the area.