An ongoing decline in the frequency of workers compensation claims has now reached larger-sized, high-cost claims, the National Council on Compensation in the US says.

Overall injury rates have dropped by an average of 3.9 percent per year since 1996 including an estimated 4.5 percent decline in 2005, according to the 2006 study. But for the two most recent years, high-cost claims have shared equally in the frequency decline.

"This is somewhat unexpected, since past studies have shown the frequency decline was much more pronounced for the smaller claims," the NCCI study said. "The decline now seems to be just as prominent among the medium and large claims as it is for the small claims."

Small workers compensation claims are those less than $2,000, large claims are those valued at $50,000 or more.

The decline in larger claims has helped dampen claim severity increases, the NCCI reports. But increases in average medical costs per claim remain substantial, averaging 9.1 percent per year over the last four years.