"Just the facts, ma'am," was all that Sergeant Joe Friday wanted from witnesses as he carried out his detective duties on "Dragnet." And most Post readers want the same.

Some days a news story's essential five W's -- who, what, where, when and why -- are hard for readers to come by.

I am not talking about "delayed leads," where readers first have to wade through mood-setting or an anecdote before finding out what the story is all about, or the occasional sports story that stirs a fudge of color and analysis before disclosing the score. I'm talking about missing facts, period.

A few recent examples:

Two days after the election, The Post carried a long story, complete with picture, about Carol Schwartz winning an at-large seat on the D.C. Council -- without once identifying her as a Republican. A separate story, 13 pages farther back, started out with "Republican Carol Schwartz," but how about readers who didn't see both?

The same day, readers were told of a University of Virginia hospital to be built, but with one key fact unstated -- where?

The next day, there was a front- page story about Rep. Frank McCloskey (D) winning reelection in a close race. But what state? Some reader curiosity was satisfied when the next day a correction identified the state an Indiana, but some of the other omissions were not addressed.

After this flurry, the Department of Missing Essential Facts went into hibernation, but a story Nov. 13, "Ashes Believed Cause of Layhill House Fire," was replete with details about the house structure, location, damage and how the fire probably occurred, but gave no information about who lived there. And none the next day, either.

Readers understand the difficulties in obtaining information in emergencies and at odd hours, but newspapers come out day after day, and blanks can be filled. Better late than never.

Readers sometimes show signs of slippage, too. A caller complained that The Post let her down in its election coverage by not alerting her during the campaign about the interest of young people in registering as Republicans and voting for President Reagan.

From The Post's files, I found a folder bulging with news stories, editorials, polls and commentaries on the subject. A few samples:

A Sept. 11 editorial, entitled "Who's Registering?" noted that "in the polls young Americans have been among the groups most heavily in support of Ronald Reagan."

A few weeks later, staff writer Bill Peterson reported from the Ohio State University campus in Columbus that "There is an unlikely new hero on campus here this fall. . . . His name is Ronald Reagan. He is 73 years old, a conservative and the very antithesis of the counterculture that dominated campuses a dozen years ago."

Two weeks later, Michael Barone, a member of the editorial page staff, did an analysis on "Why the Young Are Going for Reagan," and Oct. 28, the Outlook section devoted a half-page to two articles by college men on why the young generation was supporting Reagan.

When The Post arrives on the doorstep with words enough to fill the average hard-cover book, readers cannot be expected to note every word, but editors must wonder what more they can do to deliver signs of trends more effectively.

Photographers sometimes have similar questions. A reader called to complain that photographs taken at the Vietnam Memorial ceremonies last week did not show blacks in attendance. She had not been present, but she suspected an effort to deprive blacks of their just honors.

The Post photo assignment editor, Craig Herndon, a black himself, reacted vigorously. His photographers reported that few blacks were present and he felt it would be unethical to assign a photographer to put a black into a picture in order to satisfy a reader who suspected he was motivated by discrimination rather than professionalism. A front-page story last Sunday noted that many blacks purposely avoided the ceremonies.

Sergeant Friday had a tough job getting the facts, and so do many journalists, but some can do better -- and so can some readers.