The other day, this letter to the editor arrived from a Chris Wallace of Alexandria:

"Nothing shows the contempt of your editorial policies for the true public opinions of this country more than your treatment of the letters about Col. North. How many favorable letters did you have to throw away before you found the negative ones you printed?

"Of the eight letters printed about Col. North in your Wednesday July 15 edition, five were unfavorable. So I guess that means for every 10 letters you received concerning Col. North, six were unfavorable. Who are you trying to fool?"

Nobody, actually, though Wallace's question is a good one; others too have called or written to take issue with the particular selection of letters July 15.

"Entries . . . did not reflect public opinion as indicated by numerous polls conducted about Lt. Col. North's testimony," commented Albert Maruggi of Washington.

Wallace and Maruggi assume that letters columns, like the polls, are a measure of popular opinion. But they aren't. Polls reflect the views of a statistically significant random sample; letters, on the other hand, reflect the opinions of a self-selecting group.

While 64 percent said they had a "favorable opinion" of Oliver North in a Post/ABC poll conducted July 21, about the same percentage of letter writers to The Washington Post were saying -- in so many words -- just the opposite. Of the scores of letters received in the three days following North's testimony -- the days when most of the mail about North was received -- more than two-thirds expressed an unfavorable opinion of North and his role in the Iran-contra affair. Of 130 letters that could be categorized easily as either favorable or unfavorable, 39 were favorable, 91 unfavorable.

Some of the unfavorables were as succinct as Herb Golden's of Bethesda: "Please forward this to Lt. Col Oliver North for inclusion in his negative rating pile: Oliver North is no hero." Others were longer reflections on North the witness, North the foreign-policy maker, North the Marine, etc. Many found less wrong with the man himself than with what they perceived to be his views on democracy and the Constitution. "I wish to register an emphatic voice that does not join in the general adulation of Lt. Col. Oliver North. He is certainly bright, articulate, sincere and dedicated -- but not to the basics of democracy, the rule of law or the tenets of the Constitution," wrote Margaret M. Conant of New Carrollton.

Among the favorables were letters like the ones from David C. Tocus of Gaithersburg ("North is the guy we thought we were voting for when we voted for Reagan") and Jeanne D. Contarino of Berwyn ("I wish to state that I support Lt. Col. Oliver North in his endeavor to help release our hostages, get a better relationship with Iran and most of all support the Nicaraguan contras with both military arms and humanitarian supplies. . .").

Others in the "favorable" pile, however, were as much about the press as about Oliver North. "The level of mockery, line by paragraph by article, exhibited by The Post before and during Lt. Col. North's testimony . . . was, given the policies of the paper, expected," wrote Ann Sheridan of Washington. "You were not alone in this," she continued. "We had only to turn to our television sets prior to the opening rounds of his testimony to be assured this man would be the loose cannon that would sink the ship of state. Well, ladies and gentlemen, the media have, at long last, been hoist on their own petard." She went on to praise North for his "character, intelligence and dedication."

The mix of letters in The Post's mailbag was evidently not so very different from that received at other newspapers across the country. Letters editors at the Chicago Tribune, the Los Angeles Times and The New York Times all reported more mail against North. USA Today said the mail is now running 50-50 after an initial flurry of mail in North's favor.

According to surveys by the Gallup polling organization, letter writers are more articulate, more involved in public affairs and more politicized than people who don't write. Also "people who hold intense attitudes tend to write," says Andrew Kohut, president of Gallup, "and intensely against generates more mail."

That seems to have been the case here.

The writer is a member of the editorial page staff.