Calculation of injury frequency rate
The injury frequency rate describes how many disabling injuries occur per
1,000,000 hours of exposure across a workforce. It is a standardized indicator, which means it allows comparison between factories of different sizes or between different time periods even when the total hours worked are not the same.
The frequency rate is calculated by taking the number of disabling injuries, multiplying by 1,000,000, and dividing by the total hours worked by all employees.
In this garment factory,
800 workers each worked an average of
1,900 hours during the year. The first step is to determine the total person-hours of exposure:
Total hours worked =
800 ×
1,900 =
1,520,000 hours
The factory recorded
19 disabling injuries. Applying the formula:
Injury frequency rate = (
19 ×
1,000,000) ÷
1,520,000 =
12.5 injuries per
1,000,000 hours worked
The workdays lost (
380) are not used in this formula. That figure belongs to the
severity rate, which measures the average number of workdays lost per disabling injury or per standard hours worked.
Watch out! The frequency rate counts the number of injuries, while the severity rate measures the time lost because of those injuries. Mixing the two is a common error in occupational health calculations.
The frequency rate and severity rate are the two most widely used safety performance indicators in occupational health practice. They are applied across industries to monitor trends, evaluate the effectiveness of safety programs, and identify areas needing intervention. However, calculation methods can vary by country or reporting system, so the nurse must verify which formula is required by local regulations or company policy.
In a fabric-dyeing area, workers may be exposed to chemical hazards, wet floors, and moving machinery, all of which can contribute to disabling injuries. A frequency rate of
12.5 per
1,000,000 hours provides a baseline for the occupational health nurse to track whether safety interventions are reducing injury occurrence over time.
| Component | Frequency rate | Severity rate |
|---|
| What it measures | Number of disabling injuries | Workdays lost due to injuries |
| Numerator | Disabling injuries | Total workdays lost |
| Denominator | Total hours worked | Total hours worked or number of injuries |
| Unit | Injuries per 1,000,000 hours | Days lost per standard unit |
Key point! When the question provides both the number of injuries and the number of workdays lost, identify which indicator is being requested before selecting which numbers to use. The frequency rate uses only the injury count, while the severity rate uses only the days lost.