Risk matrix and fault tree probability

0:00 Intro

Risk matrix and fault tree probability, for the CSP exam. Every rule and number here is cited to its primary source, on screen.

0:10 Learning objectives

In this lesson, you'll learn to: read a Risk Assessment Code such as 1A, match High, Serious, Medium and Low to RACs, compute an AND gate probability, and compute an OR gate probability for independent events.

0:27 Risk assessment codes

The military system safety standard expresses assessed risk as a Risk Assessment Code, or RAC. A RAC combines one severity category with one probability level.

0:55 Reading a RAC

Both elements are always required: severity alone is not a RAC, and probability alone is not a RAC. Table III then assigns a risk level of High, Serious, Medium or Low to each RAC. Take the example in the standard: a RAC of 1A.

1:21 Fault Tree Handbook

The first character is the severity category: 1 is Catastrophic. The second is the probability level: A is Frequent. Then Table III gives the risk level for that RAC. Exam questions often ask you to name the two elements, so keep the order straight: severity first, probability second. Fault trees combine event probabilities through gates.

1:59 Worked example: AND gate

The Fault Tree Handbook gives both formulas. For an OR gate with independent events A and B: P(Q) = P(A) + P(B) − P(A)P(B). For an AND gate with independent events: P(Q) = P(A)P(B). Both formulas assume independence, meaning P(B|A) = P(B).

2:26 Worked example: OR gate

If events are dependent, these simple products do not apply, so check independence before you pick a formula. AND gate example. A mishap needs both a guard bypass and a failed interlock. 05, independent. 05. 005. For these inputs, the AND gate result is lower than either input.

2:59 Worked example: OR vs AND

Both events must occur, so adding the probabilities would be wrong. OR gate example. A fire starts if either an electrical spark or a hot-surface contact occurs. 2, independent.

3:26 On the job

3. 02. 3. Skipping the subtraction is the classic error. The overlap, where both events occur, would otherwise be counted twice. 2, independent. 6. 1. With probabilities between 0 and 1, OR raises the result above either input; AND lowers it below either input. You're the safety manager at a metal stamping plant, analyzing a crush hazard on a press. The question: how likely is the top event, and how do you express the risk?

4:10 Common exam traps

1, independent. 005. 145. Then map that probability to a probability level, pair it with the severity category to form the RAC, and let Table III assign the risk level: High, Serious, Medium or Low. Now, six traps that catch people on exam day.

4:44 Recap quiz

One: a RAC is severity and probability together. Two: the OR gate subtracts the product P(A)P(B). Three: the AND gate multiplies. Four: the formulas assume independent events. Five: Table III assigns the risk level to each RAC. Six: software risk cannot rely solely on severity and probability. Question 1. In the military system safety standard, a RAC of 1A combines which two elements? A: Frequent severity, Catastrophic probability. B: Catastrophic severity, Frequent probability. C: Catastrophic severity, Remote probability. D: Marginal severity, Frequent probability. B. The first character is the severity category, the second the probability level. Question 2. 2 feed an OR gate. What is P(Q)? 02. 30. 28. 12. 28. 02. Question 3. 1, independent. What is the probability? 15. 005. 145. 05. 005. 1. Want to see where you stand on every CSP domain? com.

25 questions, free, no card. You get a readiness score for each CSP domain.