Mid-market companies on average were charged substantially higher premiums for group health insurance coverage in 2012 than larger firms, according to a study released Monday by Automatic Data Processing Inc.

Companies with between 1,000 and 2,499 full- and part-time employees were charged an average $10,351 per employee in 2012, 16% more than employers with more than 5,000 or more workers and 8.3% more than employers with between 2,500 and 4,999 workers, according to ADP's study. The average annual per-employee premium cost across all employers surveyed was $9,562.

Chris Ryan, ADP's vice president of strategic advisory services and co-author of the study, said in an email that several factors are likely to blame for midsize employers' cost disadvantage.

Midmarket firms “lack the size and scale needed to negotiate more favorable agreements with health plans, networks and TPAs. They are also less likely to operate self-funded health plans with the potential to reduce premiums,” Mr. Ryan said, adding that recent ADP research also suggested that “larger employers are more likely to have the financial wherewithal to provide employees with a health and wellness program to help contain premium costs.”

“Larger employers may be more effective in communicating and implementing consumer-driven health plans, and may also have more effective practices for conducting audits to ensure member eligibility,” Mr. Ryan said.

ADP's “2012 Study of Large Employer Benefits,” the first of its kind for the Roseland, N.J.-based payroll processing firm, examined health care costs and participation rates among 300 employers with more than 1,000 employees. In total, the study encompassed more than 2 million covered lives, including employees and their dependents.

Beyond headcount, certain other demographic factors were found to be predictive of higher-than-average health benefit costs. Manufacturers, professional and scientific services firms, health care providers and information technology firms were charged between 3% and 13% per employee per month more for group health benefits than the average employer, according to ADP's study.

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In part, those industries' higher health care costs were driven by the average age of their respective workforces. Manufacturing companies were charged $899 per employee per month in premiums, the highest of any industry surveyed, and had an average employee age of 45.5 years. Conversely, employers in the hospitality and food services industry incurred the lowest monthly per-employee premiums in 2012 ($596) and the lowest average employee age (36.9 years) of any industry group surveyed.

Other factors such as geographic location and the “richness” of the benefits provided contributed to the disparity in benefits costs among industries, the study noted.

ADP also examined health care eligibility and participation rates among full- and part-time employees, in advance of certain health care coverage requirements under the health care reform act due to take effect in 2014.

Under the Patient Protection and Affordable Care Act, employers must offer qualified health care coverage for any employee who works more than 30 hours per week, or pay steep penalties if an eligible employee uses a federal subsidy to purchase health insurance through a public health insurance exchange.

Last year, only 15% of all part-time employees surveyed were eligible for health benefits through their employer, and only 8% had enrolled in coverage, compared with 88% and 77% of full-time employees, respectively.

“The gap looms large as the shared responsibility provision of the Affordable Care Act will soon require employees credited with service equal to at least 30 hours per week or 130 hours per month, to be automatically eligible for employer-sponsored health plans,” an ADP spokesman said in an email. “Potentially, this provision could create a spike in part-time employees eligible for benefits starting in 2014.”