“He’s basically tracking reservations, and we’re tracking what someone actually ate,” Wilner said of the difference between her group and SMG Delta. She added that many news organizations prefer not use data from a partisan source such as SMG Delta, which describes itself on its website as “the official media buying agency for the McCain-Palin campaign.”

There is another notable difference as well: according to an NBC News article, SMG Delta’s data includes radio ads in addition to television ads. Winn, Smart Media Group’s political director, did not return follow-up phone calls or an email seeking to confirm these differences in the two groups’ methodologies.

The vastly differing statistics from each of the leading sources on spending in the ad wars raises a basic question: What should reporters do to provide the best information to their audience?

One answer is to be precise and accurate in describing their sources of data. For example, Kantar statistics are estimates rather than exact totals, a difference that should be reflected in descriptions of them. There is also an important distinction between ad reservations and ads that have already aired, and this should be noted rather than simply referring to “TV ad spending.”

A second takeaway is that ad spending takes place in a volatile marketplace, in which rates rapidly change, many ads can be preempted and then must be refunded, and candidates and outside groups pull ads on short notice as they reallocate resources. Robin Kolodny, a political science professor at Temple University who studies campaign advertising, said a precise picture of TV ad spending will not emerge until early December when broadcasters disclose invoices that, unlike order forms, provide the costs and details of which political ads actually aired. “It’s much easier to do it after the fact then it is in real time,” Kolodny said. “That’s the real lesson.”

Finally, there is far more to be learned about campaign ad spending than simply overall expenditure totals. Unlike SMG Delta or the FEC, Kantar provides spot counts, which The New York Times yesterday described as a better measure of the ad wars than spending because it reflects an organization’s reach rather than the disparate prices that campaigns and outside groups pay for airtime. Kantar also offers a breakdown of the content of ads, which provides insight into campaign strategy and messaging. “Advertising isn’t just about the money,” said Wilner of Kantar’s campaign analysis team.

Sasha Chavkin covers political money and influence for CJR's United States Project, our politics and policy desk. He has written for ProPublica, the Center for Public Integrity, and The New York World. Follow him on Twitter @sashachavkin.