The National Rifle Association is distorting a survey that experts say already uses questionable methodology to claim that the vast majority of police don't believe background checks will reduce violent crime.

The Washington Post's website is currently displaying an NRA ad which states, “80% of police say background checks will have no effect on violent crime.”

But the poll in question, conducted by the law enforcement news portal PoliceOne, does not ask respondents whether they believe background checks will have an effect on violent crime. As Slate's William Saletan has noted, the only question in the survey that produced results similar to the ones the NRA cited was the question, “Do you think that a federal law prohibiting private, non-dealer transfers of firearms between individuals would reduce violent crime?” The bipartisan background check amendment currently under discussion in the Senate would not impact private, non-dealer transfers; it would only require background checks for commercial sales.

Moreover, the survey's methodology raises questions about its results. It is not a random sample, but rather a survey completed by the 3 percent of registered current and former law enforcement officers who are members of PoliceOne and chose to respond.

Academic polling experts who Media Matters contacted said this approach is questionable, because the self-selection of respondents can bias the sample. University of Michigan Professor Michael Traugott, for example, told Media Matters that “one issue that would be of particular concern is that the survey was completed by self-selected respondents,” which the Public Opinion Quarterly editor said could have skewed the results.

Other experts highlighted that the survey examines only members of PoliceOne. “How representative of all police officers are PoliceOne's members to begin with?” said Columbia University professor Robert Shapiro, a co-author of award-winning books on public opinion research. “And how does the sample compare with all police officers demographically?” Temple University's Christopher Wlezian, another editor at Public Opinion Quarterly, likewise commented that the problem with the survey is “the fact that we don't know whether the sample of respondents is representative of the population of police officers.”

“The ideal police survey would sample from all officers, not just members of any one organization,” added Columbia University's Robert Erikson, a former editor of Political Analysis.

Certainly other prominent law enforcement officials disagree with the survey. In a statement, Baltimore County, MD, Police Chief Jim Johnson, who is the chair of the National Law Enforcement Partnership to Prevent Gun Violence, responded to the NRA ad by stating: