Single Loss Expectancy (SLE)
Single Loss Expectancy (SLE) is the expected financial loss from a single occurrence of a specific risk event, calculated as asset value multiplied by exposure factor, serving as a component of the classical Annualized Loss Expectancy calculation.
The SLE Formula
Classical SLE is calculated as asset value times exposure factor, where exposure factor is the percentage of asset value lost in the event. If an asset is worth $1M and a specific event would destroy 40% of its value, SLE for that event is $400K.
Multiplied by the annual rate of occurrence, SLE produces Annualized Loss Expectancy (ALE). The two metrics have historically been paired throughout classical information security risk literature.
Where SLE Falls Short
SLE assumes a single deterministic loss magnitude per event. Real cyber events do not work that way. The same event type can produce very different losses depending on circumstances, timing, response effectiveness, and secondary consequences. A single point estimate collapses that variation.
Modern CRQ uses severity distributions rather than SLE point estimates, capturing the full range of possible outcomes for each event type. Point estimates are useful for basic reasoning and communication, but not for defensible enterprise reporting.
When SLE Still Comes Up
SLE remains in use for teaching risk fundamentals, for regulatory contexts that specifically reference the classical framework, and as a back-of-the-envelope reasoning tool. Most mature CRQ programs describe events in distribution terms while acknowledging the historical SLE/ALE lineage.
Related Terms
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