The variation of accident incidence risks prevailing in different industries or professional and other such groups can be measured by taking the number of accidents as a proportion to the number of hours worked in each branch. When this is done, the ratios gained can be regarded as the "risk figures" of a certain occupation or industry. In practice, accident proneness also varies within the same occupation or industry by task, for instance.
Accident frequency refers to the ratio of accidents and hours worked. The ratio is calculated per million hours worked. The frequency is used mainly in comparisons between different industries.
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