SOMETIMES POLLSTERS, instead of asking people only their opinions or attitudes, include questions about simple matters of fact, such as if it is the Democrats or Republicans who have a majority of seats in the Senate or House of Representatives, or what part of the world El Salvador is in.

The questions are almost always on that level of difficulty -- one step up from the old radio comedy stumper, "Who's buried in Grant's tomb?"

The scheme is to sort out people who have at least minimal knowledge of a subject from those who don't, distinguishing an "elite" group for special analysis, a class of Americans whose opinions give the appearance, however fragile, of being influenced by facts. It seems to be a rational effort, practiced with some success by the Gallup Poll, among others, for generations.

But there is almost always one unsettling finding that comes from these tests: Most people fail every one. Few Americans seem to know anything at all. The public seems to be deliberately turning away from public affairs.

Only one-quarter of the population, for example, knew, when asked in Washington Post-ABC News polls, that the Republicans have a majority in the Senate and the Democrats a majority in the House.

When CBS and The New York Times asked about the location of El Salvador, 25 percent got it right. That's an "F."

In the fall of 1981, The Post-ABC News poll asked this stickler: "Do you happen to know which two nations have been involved in what are known as the SALT talks, or is that something you are not familiar with?"

Thirty-seven percent had the correct answer, the United States and the Soviet Union. Another 13 percent gave a wrong answer, and the remaining 50 percent said they didn't know.

In the same survey, Post and ABC interviewers asked, "One of these two nations, the United States or the Soviet Union, is a member of what is known as the NATO alliance. Do you happen to know which country that is, or are you not sure?" Through random guessing -- say by flipping a coin -- 50 percent could be expected to get that one right. But the public didn't do that well.

In all, 47 percent had the correct answer, the United States. What's worse, only 3 people in 10 people had the correct answer to both the SALT and NATO questions.

Another test question in The Washington Post-ABC News poll:

"Can you tell me which two nations, aside from the United States, were involved in the Camp David peace talks?"

That one was first put to a representative national sample last March. If your guess is Israel and Egypt, or if you give the answer without guessing, then you are correct. You are also in a minority.

In all, only 45 percent knew the answer then. Another 2 percent knew that Egypt took part in the talks but they didn't know about Israel; 10 percent said Israel but not Egypt. The rest either offered no choices (36 percent) or guessed both nations wrong (7 percent).

A few months later, in August, after the Israeli invasion of Lebanon had put renewed attention on the Middle East, The Post and ABC asked the question again, of a new group. To make it a little easier, a phrase was added:

"Can you tell me which two nations, aside from the United States, were involved in the Camp David peace talks when Jimmy Carter was president?"

The addition, or, as seems more likely, the passage of time, only made matters worse. Thirty-six percent got the answer right. Then, in September, after the massacre of Palestinians by Lebanese had focused even more attention on the Middle East, the question was used again, with the previous month's phrasing. Once more, 36 percent answered correctly.

In this instance, the "special analysis" aspect of the test provided striking findings. As reported in The Post after one of the surveys, the "elite" 36 percent tended to be consistently min ore pro-Israel in their attitudes than the rest of the public, less likely to show dramatic shifts in opinion and more likely to have opinions on most issues. Whether people had at least minimal knowledge of the Middle East, in other words, seemed to have bearing on their opinions.

Often, however, there is no such relationship between awareness and attitudes. On the questions dealing with which political party controls each branch of Congress, the pattern of knowledge persists: men know the answers more than women do, whites more than blacks, and so on. But knowing the answers has hardly any bearing on which candidate or political party a person will vote for, or, indeed, on whether a person is likely to vote at all.

The results of all these tests are obviously disturbing for the overall public ignorance they reveal. Even more discouraging, though, is the glimpse they offer of groups within the population.

Taken women, for example. On every question, they did far worse than men. A majority (56 percent) of men knew the answer to the Camp David question the first time around, but only one-third (34 percent) of women did. The second and third times, about half the men answered it correctly but only one-quarter of the women did.

As might be expected, people with a college background did much better on all questions than those with less education; whites did better than blacks, and Republicans somewhat better than Democrats. (Whites and Republicans, of course, have generally received more education than blacks and Democrats.)

What do findings like these seem to be telling us?

Whatever the reasons, one thing seems clear: Whether they realize it or not, people are intentionally turning away from public affairs. It is not a lack of brains that is involved here. The little poll quizzes do not measure intelligence; they measure the storage of bits of information. A person need not be bright to avoid telling an interviewer, as one did, that El Salvador "is in Louisiana, near Baton Rouge."

At one point or another, almost every citizen who watches TV or picks up a newspaper had to have known where El Salvador is, who was in the SALT talks, and all the rest. And it had to take some effort to forget all that.