It’s still too early to know whether the job creation tax credit signed into law last March is having any effect, Treasury Department officials said on Monday.

This tax program, part of the Hire Act of 2010, is intended to encourage businesses to add workers who have been out of a job for at least 60 days by making it cheaper to employ them.

Usually, the federal government collects Social Security payroll taxes on salaries, amounting to 12.4 percent of every employee’s wages (up to $106,800; any wages over that amount are not subject to Social Security taxes). Half of this payroll tax, or 6.2 percent, comes from the employee, and the other half, or another 6.2 percent, is collected from the employer.

Under the Hire Act, businesses that hire the long-term unemployed do not have to pay this 6.2 percent tax on each worker for the remainder of 2010. Additionally, if these new employees stay on for a year, the employer gets another tax credit of $1,000.

So, for example, if a company hires a new worker at a salary of $40,000, normally the employer would have have to pitch in a payroll tax of $2,480 (6.2 percent of $40,000). But under the Hire Act, if that new employee had previously been out of work for at least 60 days, the employer doesn’t have to fork over that $2,480. Plus, if the employee stays for at least a year, the company gets the additional $1,000 tax credit.

All of this makes it cheaper to hire new people, something employers may be skittish about doing given the uncertain economic environment. It’s a relatively small amount of money at stake, when you consider the overall costs of taking on another employee, but theoretically it may be just the incentive that employers on the fence about expanding may need.

The challenge, of course, is making sure that the tax credit actually induces hiring, rather than just being claimed for people who would have gotten jobs anyway.

The Treasury Department is now trying to assess how well the program is working, and is finding it a challenging task.

In a report and a conference call on Monday, the Treasury Department reported that, from February to May of 2010, about 4.5 million workers who had been unemployed for eight weeks or longer were hired by employers, and would therefore qualify for the payroll tax exemption. If every employer that hired one of these unemployed workers claims the exemption, businesses would be entitled (collectively, of course) to $5.1 billion in payroll tax savings, Treasury officials estimated. And if every one of those hires stays on for 52 weeks, employers would receive an additional $3.4 billion in tax savings.

These calculations actually don’t tell us very much, though, because the government does not know how many employers that hired workers would eventually claim the related tax benefits.

More importantly, no one knows whether any of those tax benefits were actually the cause of the new hires, or were just happily discovered after a hiring decision had already been made and therefore had no immediate effect on job market conditions.

Andy Warlick, president and chief executive of Parkdale Mills, a yarn manufacturer in North Carolina, had been brought onto the conference call this morning by Treasury officials as an example of an appreciative employer who had recently hired 30 workers eligible for the Hire Act tax exemptions. But when asked by a reporter about whether the tax benefit was actually responsible for his decision to expand his staff, he said it had not.

He said, of course, that he was still pleased to be able to claim the credit, though, since it “lowers our cost of operating here” in the States, and may help his company decide to continue investing in America rather than moving some operations abroad.

One reason to be concerned that other companies might also claim the tax benefits only after deciding to hire, rather than having the Hire Act directly induce hiring, is that many businesses do not seem to know that the tax exemption even exists. Spreading the word about the tax credit was apparently one major goal of a Treasury Department conference call about the Hire Act this morning, said Alan B. Krueger, the Treasury’s chief economist.

The Obama administration is trying to extend the act, which expires at the end of the year, in the hopes that it will still be around by the time word actually spreads.