Republican presidential candidate Ben Carson doesn't tend to get much speaking time at debates. But at the GOP debate in Houston on Feb.25, Carson managed to get in some memorable lines. (CNN)

Ben Carson’s debate night in Houston can be summarized in one line: “Can somebody attack me, please?”

Okay, his “fruit salad” line was pretty good, too, but it was his plea for negative attention that perfectly captured his irrelevance. He was so desperate for a chance to speak that he figured a verbal assault from one of his opponents — which, by rule, would entitle him to a response — might be the only way he’d get to talk.

CNN, which broadcast the debate and supplied the moderator, Wolf Blitzer, didn’t even pretend that the retired neurosurgeon is still a factor in the Republican presidential nominating contest. Carson received just six questions in more than two hours and got only 11 minutes and 10 seconds of speaking time — roughly half the allotment of Ted Cruz and about a third of Donald Trump’s share, according to a tally by Politico.

He was basically invisible.

Managing talk time is always difficult, especially in a debate as fractious as Thursday’s. Trump, Cruz and Marco Rubio bickered, interrupted and shouted over one another constantly. But it’s philosophically challenging, too: Should a moderator try to grant equal time to every candidate or focus more heavily on the leaders?

It depends. In November, when I interviewed Fox Business Network anchors Neil Cavuto and Maria Bartiromo before they moderated a GOP debate featuring eight candidates, they acknowledged some imbalance is inevitable but said they would attempt to dole out time more or less evenly.

Their efforts made sense at that stage of the race — three months before the start of primary balloting, a point where polls aren’t very good predictors. On the day of that debate (Nov. 10), Trump and Carson were in a virtual tie atop the Republican field, according to the Real Clear Politics national average.

Carson, of course, has plummeted since then; it was just too early for moderators to judge who was or was not legit and to hand out speaking time accordingly.

But by now, it’s very clear that Carson has no shot to be the Republican nominee. While each of the other remaining candidates has managed to finish second or better in at least one of the first four primary states, Carson hasn’t placed higher than fourth. Those are real results — not polls. He hasn’t done well in the Midwest, Northeast, South or West. He has a very small constituency that is sticking by him, but there is zero reason to believe he's got a shot.

It’s hard to understand why he’s still in the race, in fact.

Debate moderators should be slow to dismiss candidates, allowing for the possibility of an improbable comeback. Perhaps that’s why John Kasich, whose own viability is in serious doubt, got almost exactly as much speaking time as Rubio.

But Carson is so far out of the running that it would have been difficult to justify giving him much air. This was the last debate before Super Tuesday. The purpose was to help voters in the upcoming states — and elsewhere — choose among the candidates who could actually represent the Republican Party in the general election.

Carson just isn’t one of them. And every minute devoted to him was a minute deducted from a real contender. This kind of thing is a judgment call, and CNN got the judgment right.