### Definition(s)

#### Likelihood

The chance of something happening, whether defined, measured, or estimated objectively or subjectively or in terms of general descriptors (such as rare, unlikely, likely, almost certain), frequencies, or probabilities.

*Source: API RP 781 Security Plan Methodology for the Oil and Natural Gas Industries.1 ^{st} Ed. September 2016. Global Standards*

#### Likelihood

The probability of a specified outcome (consequence) of an activity actually or potentially occurring.

*Source:** IOGP Report No. 510, Operating Management System Framework for controlling risk and delivering high performance in the oil and gas industry, International Association of Oil & Gas Producers, June 2014. **Global Standards*

#### Likelihood

Chance of something happening.

[SOURCE: ISO

*Source: ISO/IEC 27000:2014, Information technology — Security techniques — Information security management systems — Overview and vocabulary, Third Edition, January 2014. **Global Standards*

#### Likelihood

Chance of something happening, whether defined, measured, or estimated objectively or subjectively or in terms of general descriptors (such as rare, unlikely, likely, almost certain), frequencies, or probabilities. Likelihood of the act is a function of two subcomponents, *L*_{1} and *L*_{2}.

*Source:API STANDARD 780, Security Risk Assessment Methodology for the Petroleum and Petrochemical Industries, First Edition, May 2013. Global Standards*

#### Likelihood

The probability or frequency that a particular undesirable event will occur (AS/NZS 4360). E.g. The likelihood of being exposed to harmful levels of noise and/or ototoxins.^{
}

*Source: NOPSEMA Guidance Note: Noise Management—Principles of Assessment and Control, N-09000-GN0401, Australia, Revision 3, December 2011. Regulatory Guidance*

#### Likelihood

Chance of something happening, whether defined, measured or estimated objectively or subjectively, or in terms of general descriptors (such as rare, unlikely, likely, almost certain), frequencies, or probabilities.

**Sample Usage:** The likelihood of natural hazards can be estimated through the examination of historical data.

**Annotation:**

- Qualitative and semi-quantitative risk assessments can use qualitative estimates of likelihood such as high, medium, or low, which may be represented numerically but not mathematically. Quantitative assessments use mathematically derived values to represent likelihood.
- The likelihood of a successful attack occurring is typically broken into two related, multiplicative quantities: the likelihood that an attack occurs (which is a common mathematical representation of threat), and the likelihood that the attack succeeds, given that it is attempted (which is a common mathematical representation of vulnerability). In the context of natural hazards, likelihood of occurrence is typically informed by the frequency of past incidents or occurrences.
- The intelligence community typically estimates likelihood in bins or ranges such as “remote,”
- “unlikely,” “even chance,” “probable/likely,” or “almost certain.‖
- Probability is a specific type of likelihood. Likelihood can be communicated using numbers (e.g. 0-100, 1-5) or phrases (e.g. low, medium, high), while probabilities must meet more stringent conditions.

*Source: DHS Risk Lexicon, U.S. Department of Homeland Security, 2010 Edition. September 2010 Regulatory Guidance*

**Likelihood (Statistical)**

Conditional probability of observing a particular event given the hypothesis under consideration is true

**Sample Usage: **Analysts evaluated the likelihood of a breach in the border fence given their observations of population increases in area cities.

**Annotation:**

- Likelihood is used colloquially as a synonym for probability.
- In statistical usage there is a clear distinction between probability and likelihood: whereas probability allows us to predict unknown outcomes based on known parameters, likelihood allows us to estimate unknown parameters based on known outcomes.
- The probability of a successful attack occurring can be broken into two related quantities: the probability that an attack occurs (which is a common mathematical representation of threat), and the probability that the attack succeeds, given that it is attempted (which is a common mathematical representation of vulnerability). In the context of natural hazards, probability of occurrence is typically informed by the frequency of past incidents or occurrences. These probabilities are often colloquially referred to as likelihoods.

*Source: DHS Risk Lexicon, U.S. Department of Homeland Security, 2010 Edition. September 2010 Regulatory Guidance*

#### Likelihood

Chance of something happening.

- NOTE: 1 In risk management terminology, the word “likelihood” is used to refer to the chance of something happening, whether defined, measured or determined objectively or subjectively, qualitatively or quantitatively, and described using general terms or mathematically [such as a probability (3.6.1.4) or a frequency (3.6.1.5) over a given time period].
- NOTE: 2 The English term “likelihood” does not have a direct equivalent in some languages; instead, the equivalent of the term “probability” is often used. However, in English, “probability” is often narrowly interpreted as a mathematical term. Therefore, in risk management terminology, “likelihood” is used with the intent that it should have the same broad interpretation as the term “probability” has in many languages other than English.

*Source: ISO Guide 73:2009(E/F), Risk Management – Vocabulary, First Edition, 2009. Global Standards*