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OSHA discusses its incidence rates formulas [US]

In an interpretation letter dated August 23, 2016 the Occupational Safety and Health Administration (OSHA) detailed its formula to calculate incidence rates of occupational injuries and illness in response to concerns that the formula used was biased against small employers.

Smaller establishments’ incidence rates are affected much more by a single injury or illness compared to larger establishments, and any analysis comparing rates must consider this. Incidence rates are more meaningful when compared to other employers doing similar work, with similar workforce sizes. The Bureau of Labor Statistics (BLS) publishes incident rates by establishment size.

OSHA incidence rates formulas

The size categories are:

  • 1-10 employees
  • 11-49 employees
  • 50-249 employees
  • 250-99 employees
  • 1,000 or more employees.

Small establishments can also aggregate multiple years of data, which would minimize the effect of a single injury or illness on the incidence rate.

Finally, OSHA believes that injury and illness data is only one piece of data that should be considered when evaluating the effectiveness of a safety and health program. OSHA believes in both leading and lagging indicators.

Lagging indicators track exposures and injuries that have already occurred, while leading indicators reflect the potential for injuries and illnesses that have not yet occurred.

Some leading indicator examples include the level of worker participation in safety program activities, the number of hazards and near misses, the number and frequency of management walkthroughs, the number of days to take corrective action, or the number of workers who have completed required safety and health training.


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