Gay-rights activists are distressed over the results of the first large-scale federal survey measuring sexual orientation in the United States, in which it reported in July that less than 3 percent of the population identifies as gay, lesbian or bisexual.

They contend it is a gross undercount and are particularly upset because they worked for years to get sexual orientation added to the 57-year-old National Health Interview Survey (NHIS), the government’s premier measure of Americans’ health statuses and behaviors. Now, the activists worry that the results will reduce the urgency of their causes and give fodder to their political foes.

“The truth is, numbers matter, and political influence matters,” said Scout, director of the nonprofit CenterLink’s Network of LGBT Health Equity, who goes by only one name. “If we really are 2 percent vs. 4 percent, it means people are going to say, ‘Okay, I’m only going to care half as much,’ ” he said.

Scout and others believe that the survey, which is administered by the Centers for Disease Control and Prevention, is flawed. They point to other surveys, including some scholarly papers and less prominent government studies, that peg the number at closer to 4 percent. The CDC is investigating why its figures, particularly for bisexuals, differ from those in the other surveys.

The influential NHIS, which queried about 35,000 adults last year, found that 2.3 percent of the adult population identified as being gay, lesbian or bisexual. Of those surveyed 1.6 percent labeled themselves as gay or lesbian, and 0.7 percent identified as bisexual. Another 1.1 percent responded that they were “something else,” did not know their sexual orientation or declined to say, and 96.6 percent said they were straight.

A number of socially conservative groups immediately seized on the news. The survey “confirms what we already knew, that only 1 to 3 percent of the population identifies as gay, lesbian or bisexual,” Jeff Johnston, issues analyst with Focus on the Family, a Christian nonprofit that opposes gay rights, said in a statement.

“What’s interesting is comparing that number with public perception,” he added. “The average person thinks the percentage is much higher, probably because of the high profile that entertainment, news media and other influential sources have given homosexuality in recent years.”

Data collection has long been a priority of the gay-rights movement, which has struggled with a lack of detailed information about the community’s contours even as it has made extraordinary strides in accomplishing its most high-profile goal — promoting same-sex marriage.

The few scholarly surveys that have tried to size up the gay population typically found that the number of people who identified as gay, lesbian or bisexual is about 3.5 percent to 4 percent, with about half labeling themselves bisexual. The overall proportion rises when the question is adjusted to ask not about identity but about sexual behaviors, attractions or experiences.

There are broad misconceptions about the numbers, however. Many Americans believe the proportion of U.S. gays to be 1 in 10 — a false figure promoted in the 1960s, drawn from a book by sex researcher Alfred Kinsey. The polling organizationGallup has found that a majority of Americans actually believe the proportion is even higher, closer to 25 percent.

For such a respected survey as the NHIS to produce such a small number is a blow, said Ellyn Ruthstrom, president of the Bisexual Resource Center in Boston. She noted that the percentage of bisexuals identified in the survey is particularly low compared with other surveys.

“It’s just going to make it harder for us when we’re going out and talking to people about the bisexual population,” she said. “We have a real hard time already with people not taking the bisexual identity seriously.”

The NHIS was a top priority for gay-rights groups — more so, even, than the census, which does not ask about sexual orientation — because it is used to guide federal decisions on health care. Studies have shown that gays are more likely than straight people to smoke, and bisexuals experience particular disparities on a variety of measures.

Some activists believe that the survey would have yielded more candid results had the questions been asked indirectly or more discreetly.

The survey was conducted through a series of in-person interviews in people’s homes. Interviewers took verbal responses and entered them into a laptop. On sexual orientation, they asked, “Which of the following best represents how you think of yourself?’’ Respondents were given the option, depending on their sex, to respond as gay or lesbian, straight, bisexual, “something else,” or “I don’t know the answer.”

For example, the 2013 National Adult Tobacco Survey, a larger but less prominent CDC study, interviewed respondents exclusively over the phone. The results more resembled what gay-rights groups had expected. It found that 3.5 percent of Americans considered themselves gay, lesbian or bisexual, with 1.9 percent labeling themselves gay or lesbian and 1.6 percent identifying as bisexual.

But the CDC conducted rigorous tests to come up with the questions and interview method for the NHIS, researchers with the CDC’s National Center on Health Statistics said. They conducted more than 100 in-depth interviews — far more than is typical — and did three field tests, including one in which they experimented with a more private interview method that allowed respondents to listen to questions using headphones and type their answers into a computer.

The researchers found no difference in the results using the two methods, said James Dahlhamer, a health statistician with the CDC. The agency is investigating why the NHIS results are so different from those of other surveys, he said.

“There was a lot of testing that went into the front end in terms of the development of the question, but we are always striving to improve our questions,” he said.

The NHIS still offers valuable insight on health issues, advocates said. But the top-line figure for the percentage of gays, which is what grabbed headlines, prompted some soul-searching. One activist is asking a provocative question: Should the gay-rights movement stop focusing so much on its numbers?

“There’s a saying within the Beltway that ‘you don’t count if you’re not counted,’ and I really contest that,” said Shane Snowdon, director of the health and aging program for the Human Rights Campaign. “We would deserve protection if our numbers were a fraction of what they are in the NHIS.”