Definitions and Formula
What kind of ergonomics numbers exist?
"Ergonomics" is a general term that has different meanings to different audiences. Most often, this term is applied to work-related musculoskeletal disorders (MSDs). The U. S. Department of Labor defines an MSD as an injury or disorder of the muscles, nerves, tendons, joints, cartilage, and spinal discs. MSDs do not include disorders caused by slips, trips, falls, motor vehicle accidents, or similar accidents. The Bureau of Labor Statistics publishes detailed characteristics for MSD cases that resulted in at least one lost day from work.
How do I compute injury rates for time periods of less than a year?
The Bureau of Labor Statistics produces annual rates, only, based on annual data, so any comparison may be inexact. As indicated in the guide, How to compute your firm's incidence rate, the basic formula is:
(Number of injuries and illnesses X 200,000) / Employee hours worked = Incidence rate where the 200,000 hours in the formula represent the equivalent of 100 employees working 40 hours per week, 50 weeks per year.
One could compute a partial year incidence rate by dividing the number of cases by the hours worked for a certain period, and then multiplying the result by the part of 200,000 (the 12-month constant) represented by that certain period. For a single month, you would use 16,667. This approach, however, assumes that your injury and illness experience grows at a constant rate for the year. The alternative is to not adjust the constant (leave it at 200,000), and this assumes that you will not experience any additional injuries or illnesses. Both assumptions may not be too realistic.