What it means
In statistics and public health, incidence counts only new cases that start in the period being measured. This is different from prevalence, which counts all existing cases, old and new, at a point in time.
If ten workers are injured this year, that is incidence, whereas the number of workers still affected by earlier injuries is prevalence. To compare groups of different sizes, the number of new cases is divided by the population at risk and often multiplied by a convenient base, such as 1,000 or 100,000.
A rate of five new cases per 1,000 people per year can then be compared across companies, regions or time periods. Without the division, a large organisation would always look worse than a small one.
In workplace safety, a widely used version expresses recordable injuries per 200,000 hours worked. That number comes from 100 full-time employees working 40 hours a week for 50 weeks, so the rate can be read as injuries per 100 full-time workers per year.
Using hours worked rather than headcount adjusts for part-time staff and overtime. Finance and insurance teams care because the rate is linked to cost.
Insurers use injury and claim rates when setting workers' compensation premiums, and a rising rate can lead to higher premiums, lost production and legal costs. Boards often include the incidence rate in their risk reports as a leading sign of control problems.
The measure also supports targets and incentives. A company might aim to cut its rate by 20% over two years, then check whether the investment in training and equipment achieved it.
The figure is only reliable when injuries are recorded consistently, because under-reporting makes a workplace look safer than it is. The nuance is that the rate measures frequency and not severity.
A business with many minor injuries and one with a few fatal accidents can have similar rates, so it should be paired with a severity measure, such as days lost per injury.
In practice
Real-world examples.
Example
A construction company records 18 injuries over 1,200,000 hours worked. Its rate is (18 x 200,000) / 1,200,000 = 3.0, which the safety director compares with an industry average to see whether the firm is better or worse.
Example
A hospital group reports 45 new infections among 15,000 admitted patients in a quarter. The incidence is 45 / 15,000 = 0.3%, or 3 per 1,000, and the group tracks it each quarter to see whether infection control is working.
Example
An insurer reviewing a logistics client finds the injury rate has doubled in a year. It raises the premium by $50,000 and requires a safety improvement plan before the next renewal.
Formula
Calculation
Incidence rate = (number of new cases / population at risk) x base
Workplace version: incidence rate = (number of recordable injuries x 200,000) / total hours worked
Suppose a factory records 6 injuries in a year, and its employees worked 400,000 hours in total. The rate is (6 x 200,000) / 400,000 = 1,200,000 / 400,000 = 3.0 injuries per 100 full-time workers. If the number of injuries falls to 4 on the same hours, the rate becomes (4 x 200,000) / 400,000 = 2.0.Case study
Seen in the real world.
Pelham Packaging is an illustrative, fictional manufacturer with 250 employees. Its board noticed the total number of injuries had risen from 9 to 14 in a year and asked whether this was a problem or just a result of growth.
The finance director calculated the incidence rate for each year. Hours worked had grown from 450,000 to 520,000, so the rate moved from (9 x 200,000) / 450,000 = 4.0 to (14 x 200,000) / 520,000 = 5.4, which showed a real deterioration beyond the effect of growth.
The board funded a $120,000 programme of equipment guarding and training. The illustrative lesson is that raw counts hide the truth, and an incidence rate based on hours or population reveals whether safety is improving or slipping.
Watch out
Common mistakes.
- Comparing raw numbers of cases between large and small organisations, when the rate must be adjusted for population or hours worked.
- Confusing incidence with prevalence, which counts all existing cases and not only new ones.
- Relying on the rate alone, when it measures how often injuries happen and not how serious they are.
Questions
People also ask.
What is the difference between incidence and prevalence?
Incidence counts new cases in a period, while prevalence counts all cases, old and new, at a point or during a period.
Why use 200,000 hours as a base?
It represents the hours worked by 100 full-time employees in a year, so the result reads as injuries per 100 workers.
Can the incidence rate be used for financial events?
Yes, the idea applies to anything that is counted as a new occurrence in a population, such as defaults per 1,000 loans in a year.
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