Even Rudy Giuliani would acknowledge that he can be prickly. Now, it seems, the tough-talking former mayor is growing estranged from empirical fact.

I'm referring to his presidential campaign's recent radio ad in New Hampshire, in which Giuliani speaks of his personal experience with prostate cancer and then cites an ear-grabbing statistic: "My chances of surviving prostate cancer -- and thank God I was cured of it -- in the United States: 82 percent. My chances of surviving prostate cancer in England: only 44 percent under socialized medicine."

Hold it, you mean I'd be nearly twice as likely to die of prostate cancer in Liverpool as in Los Angeles? Twice as likely to succumb in Oxfordshire as in Ohio? Amazing. Also, not remotely true.

As several truth-squading journalists -- notably, The Post's Michael Dobbs-- have pointed out, mortality rates from prostate cancer in Britain and the United States are roughly the same: About 25 men out of 100,000 die of prostate cancer each year in both countries. (That's the standard way of reporting mortality rates, deaths per 100,000 individuals.) I'll get to the math a little later -- that's a promise, not a threat -- but first, I want to try to understand Giuliani's thought process. Giuliani wasn't spoon-fed those dodgy figures by some speechwriter. He plucked his data on prostate cancer from an article in City Journal, a publication of the conservative Manhattan Institute, which has been crusading against the idea of single-payer health-care systems such as the National Health Service in Britain.

Campaign aides point out that City Journal is respected and that the article was written by a physician, David Gratzer. That's true, but still: If you read that you were twice as likely to die of a given disease in Cambodia than in the United States, that would be roughly consistent with what you know about Cambodia and what you know about the United States. It would make sense that advanced, lifesaving treatment would be hard to obtain in an underdeveloped country with a fairly rudimentary health-care system.

Britain, however, is hardly Cambodia. Giuliani has been to London as recently as September (it was there that he announced his status as "probably one of the four or five best-known Americans in the world"). Did it not strike him as unlikely that such an advanced society would be so callous, incompetent or both in treating a fairly common, well-studied disease such as prostate cancer? Wouldn't he want to have that statistic checked out before citing it in his presidential campaign?

I see two possibilities. One is that he believed what he wanted to believe -- that this huge supposed disparity in cancer outcomes fits so neatly into his worldview that it just had to be right. Hmmm, isn't cherry-picked data -- about weapons of mass destruction, not cancer survival rates -- the reason we have nearly 160,000 troops bogged down in Iraq?

The other possibility is that Giuliani didn't really care whether the figures made any sense or not. He invokes the specter of "Hillarycare" -- shorthand for any health-care reform that Hillary Clinton might propose -- almost as often as he reminds audiences of Sept. 11. Here was another weapon to use against his nemesis.

Okay, the math: Gratzer writes that his figures come from seven-year-old data from the Organization for Economic Cooperation and Development on the numbers of men in various countries who are diagnosed with prostate cancer and, of those diagnosed, how many die from the disease. The latest official figures show a much smaller gap: Of men diagnosed with prostate cancer, about 98 percent survive five years in the United States vs. about 74 percent in Britain.

But even that is misleading, because -- as even Gratzer acknowledges -- a much higher percentage of American men than Britons are diagnosed with prostate cancer in the first place. The reason Americans are more likely to be diagnosed is that we are screened and tested much more often than our British counterparts. Doctors here are much more likely to diagnose, say, a slow-growing tumor in an elderly patient who will die of something else before the prostate cancer progresses to a serious state.

That's why the more relevant comparison, experts say, is mortality rates -- which are about equal. For the record, I prefer our system of screening and testing; if I'm going to be hit by a freight train, I want to see it coming.

What I don't want is another president who refuses to let the facts get in the way of a good story.

eugenerobinson@washpost.com