Glossary · Risk estimation methods
Likelihood
In machine safety risk estimation, likelihood is the estimated chance that harm will occur, considering exposure and the probability of the hazardous event. It is combined with severity to estimate risk.
- Machine safety
- Risk assessment
In one sentence
Likelihood is the estimated chance that harm occurs, built from exposure, event probability and avoidance, and paired with severity in risk estimation.
Example
For an unguarded conveyor return roller that is only reachable from a maintenance platform, the team rates likelihood as low; for the same roller beside a walkway it is rated high.
How it applies
- Risk estimation: ISO 12100 builds the probability of harm from exposure, the probability of occurrence of the hazardous event and the possibility to avoid or limit harm. Rate these separately before combining them.
- Functional safety: Likelihood of harm is not the same as the failure rate of a safety function. Quantities such as PFHd describe control-system reliability, which affects the probability of the hazardous event.
- Technical documentation: Define likelihood categories in words and, where possible, with frequencies, so that ratings are reproducible across engineers and projects.
- Evidence: Field data, incident records and maintenance history can support or challenge likelihood ratings over time.
Likelihood vs. severity
Likelihood describes how probable harm is. Severity describes how bad it would be. Reducing likelihood does not change severity; a rare but fatal hazard still needs strong measures.