Sick at work and surfing the net? You're not alone -- or are you?

But researchers at University of Michigan believe those numbers may be inaccurate. A new opinion paper suggests that the tools for measuring and quantifying hours of lost productivity and translating those hours to dollars are unreliable and don't capture the entire presenteeism picture, said Susan Hagen, an analyst from the U-M School of Kinesiology Health Management Research Center (HMRC).

Because of this, the HMRC has suggested a three-year moratorium on its studies of presenteeism that translate hours of productivity loss into financial or dollar equivalents.

The HMRC defines presenteeism as reduced productivity at work due to health conditions such as asthma, back pain, allergies or depression.

"It's hard to be 100 percent effective every moment you're at work," Hagen said. "We're talking about the lack of productivity that stems from a health condition, or because you're worried about your health."

One of the challenges in measuring presenteeism is that all the measurement instruments use self-reported data. This means you're depending on employees to report they aren't working as effectively as they could be, due to their health.

"There are all kind of estimates as to how often it happens," Hagen said. "The estimates can vary so widely. Some studies say that most workers don't have any presenteeism, while there is other research that suggests most workers experience presenteeism to some degree."

Another big problem is that there are so many different measuring tools, and each tool may measure presenteeism in a different way. Also, not all health problems affect workers in the same ways. For example, a person may have allergies for two weeks in May and feel horrible, but the measurement instrument could take that two weeks and expand that bad experience to 12 months. This process vastly over-reports the illness and thus, the hours lost and the subsequent financial loss.

"Almost everybody believes in the concept of presenteeism but maybe some of those calculations based on those early measurements aren't accurate," Hagen said.

"Our concern is that organizations may be making financial or future decisions based on data that may not support those decisions," Hagen said.

The paper appears in the November issue of the Journal of Occupational and Environmental Medicine.