If your employer rewards you if you sign up to get screened for brewing health problems, count your steps, or quit smoking, you're far from alone: Workplace wellness programs are a huge business, encompassing over 50 million American workers and a nearly $8 billion industry.

It just seems to make sense: If employers help workers get healthier, that should mean lower medical costs, fewer sick days, higher productivity — an all-around win-win-win well worth the investment, right?

Well, maybe. Eventually. But a major new study finds virtually zero benefits from a workplace wellness program in its first year: No lower health costs. No fewer sick days. No extra trips to the gym. No rise in productivity.

The Illinois Workplace Wellness Study aimed to determine the causal impact of the program on health and employment outcomes, said co-author David Molitor of the University of Illinois at Urbana-Champaign. "And across 39 different outcomes that we looked at, we found zeroes — and fairly precise zeroes — on almost all outcomes," including health spending.

He noted two exceptions: Workers who joined the wellness program did become likelier to be screened for health issues, and to say they thought their employer put a high priority on employee health.

So how can this be? How can employers have flocked so enthusiastically to programs whose results — at least in the short term — are this underwhelming?

The short answer is this age-old maxim of statistical research: Methods matter.

Many of the previous studies that found impressive benefits from wellness programs were "observational," simply measuring the differences between employees who signed up for the wellness program and those who didn't.

The rub: self-selection. Workers who sign up for wellness programs are likely to be those who already lead healthier lives, and are happy to win financial rewards for things they do anyway.

To combat that bias, Molitor and his colleagues used what's considered the gold standard of research studies: a randomized, controlled trial. It assigned nearly 5,000 University of Illinois employees randomly to the wellness program or to a control group with no access to the program.

Consider two employees, Julia and Jane. Julia already enjoys going to the gym, gets regular checkups, and watches her diet. Jane is a smoker, works an extra job that deprives her of sleep, and is 30 pounds overweight.

Enter the wellness program. Which employee do we think is more likely to participate? You'd assume it's Julia.

Study co-author Damon Jones of the University of Chicago explains:

With an observational study, at the end of the year, we compare participants (mostly Julias) to non-participants (mostly Janes). If we find a difference in health and spending, it could be because Julia was healthier to begin with. With a randomized controlled trial, we avoid this concern.We take half the Julias and half the Janes and put them in a control group. Then we take the other halves and put them in the treatment group, who can access the wellness program.We know the two groups are now roughly similar to begin with. So, at the end of the year, any differences are due to the wellness program, and not pre-existing differences.

If there are any differences, that is.

The study, just out from the National Bureau of Economic Research and funded by federal and nonprofit research grants, is likely to be unpopular among workplace wellness advocates who stress the programs' benefits.

Anticipating potential pushback, the researchers did their best to ensure that the university wellness program they followed was top shelf, said study co-author Julian Reif of the University of Illinois at Urbana-Champaign. It included on-campus health screening, online assessments, a large suite of wellness activities and financial rewards for completing steps in the program.

"We don't think you can criticize it in the sense of saying, 'Well, that's not a very strong wellness program,' " he said. "We relied on industry guidelines and designed our workplace wellness program to be as effective as possible."

The study's authors do acknowledge that it may take longer for a wellness program to show results, and plan to continue their study for four more years.

"We don't have a particular stance on whether these programs work or don't work, Molitor said. "We're also open-minded regarding the possibility for effects to emerge in the long run when they don't in the short run."

The team has measured but not yet analyzed "biometric" outcomes, such as changes in blood pressure or cholesterol. That's their next task, they say.

For now, Molitor says, the study suggests that workplaces that adopt wellness programs should not expect immediate results.

Another finding: Offering a $100 reward for participating appears to be almost as good as a $200 reward, in terms of spurring participation, so that level of financial incentive may suffice.

And one concern the study raises: Smokers were among the employees least likely to participate in the wellness program, Molitor said.

"How are you going to impact their behavior if they don't show up and participate?" he said. "So these broad-stroke wellness programs may not be effective at targeting certain groups of employees who may need it the most."

Ultimately, the researchers say, they hope their study will highlight the pitfalls of typical studies on workplace wellness programs. If they had done a typical observational study with their big data set — even trying hard to offset the effects of self-selection -- they could have come out thinking the wellness program affected health, Molitor said.

"Our main hope is that this study provide credible evidence on the impacts of these programs in a space where credible evidence is lacking for how big of an issue workplace wellness is," he said.