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Annex 4 - Likelihood and Impact Scoring Scales
This chapter to be reviewed by November 2008
Likelihood Scoring Scale - Quantitative and Qualitative Measures
Level | Descriptor | Indicative Chance of Occurrence in 5-Year Period | General Description |
|---|
1 | Negligible | 0.005% to 0.05% or 20,000 up to 1 in 2,000 chance | May occur only in very exceptional circumstances. May occur with a chance of between 1 in 100,000 to 1 in 10,000 per year at most. |
2 | Rare | Greater than 0.05%, up to 0.5% or >1 in 2,000 and up to 1 in 200 chance | Very few recorded incidents or anecdotal evidence; and/or no recent incidents in associated organisations, facilities or communities; and/or little opportunity, reason or means to occur. May occur with a chance between 1 in 10,000 and 1 in 1,000 per year. |
3 | Unlikely | Greater than 0.5%, up to 5 % or >1 in 200 and up to 1 in 20 chance | Might occur at some time; and/or few, infrequent, random recorded incidents or little anecdotal evidence; and/or few incidents in associated or comparable organisations, facilities or communities; and/or some opportunity, reason or means to occur. May occur with a chance of between 1 in 1,000 and 1 in 100 per year. |
4 | Possible | Greater than 5%, up to 50 % or>1 in 20 and up to 1 in 2 chance | Regular recorded incidents and strong anecdotal or predictive evidence. May occur/ recur with a chance of between 1 in 100 and 1 in 10 per year. |
5 | Probable | Greater than 50% or 1 in 2 chance | High level of recorded incidents and/or very strong predictive evidence. Likely to occur/recur with a chance of more than 1 in 10 per year. |
Note: The likelihood scale above changes exponentially - by an order of magnitude (times 10) - per level of the scale - a logarithmic scale. This is because many of the events covered in any risk assessment will tend to be unlikely and the majority will then cluster at the lower end of a linear scale of likelihood. This makes it difficult to discriminate between event types and gives a picture of little value to planners. In addition, for many types of event it is only possible to differentiate likelihoods by orders of magnitude because no accurate statistical or historical data is available to support a more definitive analysis. The ">" symbol in the table means "greater than".
Descriptions of hazards such as "a one in one hundred year event" are equivalent to the likelihood for this scale of event expressed as a fractional or percentage probability for each year, i.e. for this example, a "1 in 100 per year" likelihood, or a "1% per year" likelihood of this scale of event happening. This is the format used in the general descriptions column of the above table. This yearly likelihood becomes 5 times greater (more likely) extended over a five-year period - i.e. for the above example - a 1 in 100 per year likelihood becomes a 5% or 1 in 20 likelihood over five years.
Impact Scoring Scale - Qualitative Measures
Level | Descriptor | Categories of Impact | Description of Impact |
|---|
1 | Insignificant | Health | Insignificant number of injuries or impact on health. |
Social | Insignificant number of persons displaced and Insignificant personal support required. Insignificant disruption to community services, including transport services and infrastructure. |
Economic | Insignificant impact on local economy. |
Environment | Insignificant impact on environment. |
2 | Minor | Health | Small number of people affected, no fatalities and small number of minor injuries with first aid treatment. |
Social | Minor damage to properties. Minor displacement of a small number of people for less than 24 hours and minor personal support required. Minor localised disruption to community services or infrastructure for less than 24 hours. |
Economic | Negligible impact on local economy and cost easily absorbed. |
Environment | Minor impact on environment with no lasting effects |
3 | Moderate | Health | Sufficient number of fatalities with some casualties requiring hospitalisation and medical treatment and activation of major accident procedures in one or more hospitals. |
Social | Damage that is confined to a specific location, or to a number of locations, but requires additional resources. Localised displacement of more than 100 people for 1-3 days. Localised disruption to infrastructure and community services. |
Economic | Limited impact on local economy with someshort-term loss of production, with possible additional clean-up costs. |
Environment | Limited impact on environment with short-term or long-term effects. |
4 | Significant | Health | Significant number of people in affected area impacted with multiple fatalities, multiple serious or extensive injuries, significant hospitalisation and activation of major accident procedures across a number of hospitals. |
Social | Significant damage that requires support for local responders with external resources. 100 to 500 people in danger and displaced for longer than 1 week. Local responders require external resources to deliver personal support. Significant impact on and possible breakdown of delivery of some local community services. |
Economic | Significant impact on local economy with medium-term loss of production. Significant extra clean-up and recovery costs. |
Environment | Significant impact on environment with medium-to long-term effects. |
5 | Catastrophic | Health | Very large numbers of people in affected area(s)with significant numbers of fatalities and large number of people requiring hospitalisation with serious injuries with longer-term effects. |
Social | Extensive damage to properties and built environment in affected area requiring major demolition. General and widespread displacement of more than 500 people for prolonged duration and extensive personal support required. Serious damage to infrastructure causing significant disruption to, or loss of, key services for prolonged period. Community unable to function without significant support. |
Economic | Serious impact on local and regional economy with some long-term, potentially permanent, loss of production with some structural change. Extensive clean-up and recovery costs. |
Environment | Serious long-term impact on environment and/or permanent damage. |
Assessment of Impact. The impact scoring should be based on the estimations of scale provided in Annex 2 or the additional threats guidance - or as determined by the appropriate Category 1 Responder(s) and UK, Scottish or other organisations, using the approach suggested in this guidance.
The impact categories are given equal weighting and the overall impact score for each hazard or threat - for use in the risk rating matrix - is obtained by calculating the arithmetic mean (total divided by 4) of the four levels scored - i.e. one score for each category. The total impact score should be a whole number (no fractions) and the decision to round the mean value up or down will depend on local judgement, informed by the local circumstances prevailing.
For example, if the scale of a particular hazard scores Level 3 for Health, Level 3 for Social, Level 2 for Economic and Level 1 for Environment, then the mean score will be 9 divided by 4 = 2.25, rounded down to an impact score of 2 on consideration by the SCG.
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