The campaign argues that giving housing and transit police officers jurisdiction beyond the city’s public housing and subways gave the city more flexibility to fight crime. It said that it usually notes the effects of the merger when describing the size of the police force, and said it would change a post on its Web site to mention the merger when citing the increase.

And the group also found that Mr. Giuliani erred at a Republican debate when, while calling for tort reform, he said that 2.2 percent of the nation’s gross domestic product “is spent on all these frivolous lawsuits.” That statistic, the group reported, came from a study that pegged the cost of all civil claims at 2.2 percent of the G.D.P., without judging whether the cases had merit or not.

Even some people who support Mr. Giuliani’s proposals say he risks undercutting his own arguments when he relies on imprecise or questionable statistics.

In a recent radio advertisement by the campaign about his health care proposal, Mr. Giuliani repeated another false statement that he had been using on the campaign trail. In the advertisement Mr. Giuliani, who has had prostate cancer, asserted that his chances of surviving prostate cancer in the United States were 82 percent, while his chance of surviving in England would have been only 44 percent. His point was that the American health care system is far superior to England’s government-run system, which he refers to as “socialized medicine.”

The figure came from an article written by one of Mr. Giuliani’s health care advisers, but was soon discredited: the source of the research that was used to derive the statistic said that its data had been misused. The Office for National Statistics in Britain said that the true five-year survival rate was 74.4 percent — still lower than in the United States, but by a much smaller margin. Mr. Giuliani stood by the statistic, however, and kept using the advertisement, though it has since gone off the air.

Ramesh Ponnuru, a senior editor at National Review magazine, said Mr. Giuliani’s plan “may be the best of the Republican health care plans.”

“The trouble is that the exact statistic he used was misleading,” Mr. Ponnuru said in a recent interview, elaborating on a blog post he wrote. “It became an argument about the statistics, and he dug in and defended it when he was wrong.”