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White House press secretary Jay Carney gestures during the daily press briefing, Monday, Nov. 18, 2013, at the White Hosue in Washington. Carney answered questions about the ongoing rollout of the new healthcare law. (AP Photo/ Evan Vucci)

The vast majority of Americans listening to the vast majority of words coming out of Jay Carney's mouth would find it a vastly confusing user experience.

The presidential press secretary has spent a week or so trying to explain what White House officials mean when they say the beleaguered HealthCare.gov website will "work smoothly for the vast majority of users" by month's end.

A "vast majority" would presumably be somewhere between a bare majority of 51 percent and an overwhelming majority of, say, 99 percent -- but officials had refused to say. My Washington Post colleagues Amy Goldstein and Juliet Eilperin solved the mystery, reporting over the weekend that a vast majority, as defined in secret by the Obama administration, is 80 percent.

Carney, at his briefing Monday, didn't deny the figure but did his best to qualify it with all sorts of asterisks and fine print about metrics and baskets and site stability and users' experiences and navigators and channels and something about a continuum. Sticking closely to his written notes for the briefing, he explained that although some of the 20 percent who couldn't get HealthCare.gov to work would have technical troubles, others might have complicated circumstances or would simply prefer to sign up on the phone, in person or even by mail.

Nedra Pickler of The Associated Press pointed out that if Amazon or Kayak couldn't serve 20 percent of their customers, "they probably wouldn't stay in business very long."

"You're looking at that statistic simplistically," Carney informed her.

CBS News' Major Garrett asked Carney for his opinion on whether 80 percent qualifies as a vast majority.

"Others can decide whether or not 80 percent is a vast majority," the spokesman said, before reverting to a discussion of "technical metrics" that are apparently very difficult to grasp. "I'm a layman here," Carney explained.

But he assured everybody that there would be "improvements to the website so that more and more Americans, up to a vast majority and beyond -- because this work will continue -- have an acceptable experience online."

Up to and beyond a vast majority?

If your head hurts, Carney is doing his job.

Much of the trouble surrounding the Obamacare rollout can be traced to a single sentence President Barack Obama voiced repeatedly: If you like your health insurance plan, you can keep it. This is not so, and White House officials knew it at the time, but Obama skipped the fine print and qualifiers.

Now Carney, determined to avoid making another promise that can't be kept, is all fine print and qualifiers.

He admitted that the administration had learned that "passage of that legislation is only half the battle, and implementation of it is just as -- in fact, more important." But he was careful not to say anything that might come back to bite him.

When asked about reports of an impending staff shake-up, Carney said there would be no "Monday morning quarterbacking here."

When CNN's Jim Acosta asked about predictions by former White House officials that staff changes would be coming, Carney repeated that "Monday morning quarterbacking is not what we're engaged in."

No, what he is engaged in is defining a "vast majority" in a way that doesn't commit the White House to a promise it can't keep.

If 80 percent is the hurdle for success, NBC's Peter Alexander asked, how would failure be defined?

"Well," the spokesman said, "I think we've said that we are making improvements so that it will function effectively for the vast majority of users."

Yes, he did mention something about that.

Dana Milbank is a syndicated columnist.