Before decisions get made in Washington -- and even before most politicians open their mouths about key issues -- there are polls. Lots of them. Whether splashed across front pages or commissioned by candidates for private analysis, the statistical sampling of public opinion is a constant in political life.

We may believe that polls tell us what Americans are thinking. But polls also gauge the effectiveness of media spin -- and contribute to it. Opinion polls don't just measure; they also manipulate, helping to shape thoughts and tilting our perceptions of how most people think.

Polls routinely invite the respondents to choose from choices that have already been prepared for them. Results hinge on the exact phrasing of questions and the array of multiple-choice answers, as candid players in the polling biz readily acknowledge.

"Slight differences in question wording, or in the placement of the questions in the interview, can have profound consequences," Gallup executive David Moore wrote a few years ago in his book "The Superpollsters." He observed that poll outcomes "are very much influenced by the polling process itself." And in turn, whatever their quality, polling numbers "influence perceptions, attitudes and decisions at every level of our society."

In the process, opinions are narrowed into a few pre-fabricated slots. The result is likely to be mental constriction in the guise of illumination.

"Opinion-polling as practiced in the United States ... presents itself as a means of registering opinions and expressing choices," media critic Herbert Schiller noted three decades ago. His assessment of polling remains cogent today: "It is a choice-restricting mechanism. Because ordinary polls reduce, and sometimes eliminate entirely, the ... true spectrum of possible options, the possibilities and preferences they express are better viewed as 'guided' choices."

Mainstream polls are so much a part of the media wallpaper that we're apt to miss how arbitrarily they limit people's sense of wider possibilities. And we may forget that those who pay the pollsters commonly influence the scope of ideas and attitudes deemed worthy of consideration.

In his book "The Mind Managers," Schiller pointed out: "Those who dominate governmental decision-making and private economic activity are the main supports of the pollsters. The vital needs of these groups determine, intentionally or not, the parameters within which polls are formulated."

When the U.S. government takes military action, instant polls help to propel the rapid-fire cycles of spin. After top officials in Washington have engaged in a well-coordinated media blitz during the crucial first hours of warfare, the TV networks tell us that most Americans approve -- and the quick poll results may seem to legitimize and justify the decision to begin the bloodshed.

In the case of the Bush administration's plans to launch an all-out attack on Iraq, the U.S. military build-up in the Persian Gulf region has run parallel to a sustained propaganda campaign on the home front during the past several months. Even so, the extent of public support is foggy.

At the end of September, a murky picture emerged from an article in the Washington Post by the director of the big-bucks Pew Research Center for the People and the Press. "Almost all national surveys this year," Andrew Kohut wrote, "have found a broad base of potential support for using military force to rid the world of Saddam Hussein." Yet such generalities can be deceiving. Kohut reported that the Pew Center's latest poll "found that 64 percent generally favor military action against Iraq, but that withers to 33 percent if our allies do not join us."

According to a recent CBS News poll, 51 percent of Americans say that Hussein was involved in the 9-11 attacks. But there's no evidence for that assertion. So, as in countless other cases, the failures of news media to clearly convey pivotal matters of fact -- and the unwillingness of journalists to challenge deceptive claims from the White House -- boost the poll numbers for beliefs that lack a factual basis.

Polls may seem to provide clarity in a confusing world. But all too often they amount to snapshots taken from slanted angles.

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Norman Solomon writes a syndicated column on media and politics.