You don’t have to agree with Fortune magazine columnist Allan Sloan‘s policy prescriptions to appreciate that his latest column exposes some of the problems associated with the corporate income tax.

Imagine being a brain surgeon given the job of fixing a tumor without being able to see its true size or shape. That wouldn’t work too well, would it? It would be even worse if you thought you knew the tumor’s size and shape but you had it wrong. Sure, that sounds ridiculous. But that’s the situation in which we find ourselves when it comes to fixing the federal corporate income tax, which has a 35% stated rate but more holes than Swiss cheese.

The only people who know for sure how much a company pays in U.S. income taxes are a few insiders. But people think they know a major reason why what should be a rational conversation gets bogged down in histrionics. …

… [T]his is the time of year that we begin to see the highly publicized but badly flawed tax studies that various groups produce by aggregating the flawed numbers in corporate 10-K annual reports, most of which are filed in late March.

There are more than a dozen different tax metrics disclosed in a 10-K — but not the federal income tax incurred for a given year. So when the likes of Paul Ryan talk about needing to cut the corporate tax rate to 25%, there’s no possible way he — or anyone — can compare that with what companies pay now.

But there’s no way that corporate critics can know either. The stories you read about disgracefully low corporate tax rates are based on the so-called current portion of taxes due, disclosed in 10-K footnotes. Many people — including me, years ago, before I learned better — use that number as a proxy for the federal income tax that a company pays. But that’s a mistake.

The current-portion number is something that companies report for purposes of calculating earnings, and it has no connection whatsoever with what a company actually forks over to the IRS for a given year.

Of course, one good way to solve the confusion surrounding the corporate income tax is to scrap it. That’s an idea Roy Cordato has proposed at the state level.