Lessons · Cybersecurity · single loss expectancy
What one bad day costs: single loss expectancy
Single loss expectancy is what you lose if it happens once: the value of the asset multiplied by the fraction of it you lose, called the exposure factor.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
The board will not fund a control because it feels risky; it funds one because somebody put a number next to it. This is the first number, and every other figure in risk work is built on it.
How to think about it
Write the asset value first, in money, however rough. Then ask what fraction would be lost in one event: all of it, half of it, a quarter. That fraction is the exposure factor, between 0 and 1. Multiply, and keep the units.
Worked example
SLE = asset value * exposure factorThe one line.
SLE = 200,000 * 0.25 = 50,000A 200,000 dollar customer database, a quarter of it lost in one event.
The exposure factor is how much is lost, not how likely it isLikelihood comes next, and mixing the two is the usual mistake.
Write the units: 50,000 dollars per event, not per yearPer event. The year arrives in the next lesson.
Your turn
A van worth 60,000 dollars, half of it lost in one event. Write the line that gives the SLE.
SLE = 60000 *
Solve one, graded on the server
The trap
Putting the chance of it happening into the exposure factor. An event that destroys everything has an exposure factor of 1 even if it happens once a century.