1 means the highest priority, but the largest value under guideline (1) is Z with 300, not W with 100. Even multiplying W's reducible impact amount of 500 by its probability of occurrence of 0.3 only gives 150, which does not reach Z, whose impact amount gives 5,000 × 0.1 = 500. Priority 1 goes to Z, so 1 does not belong in a.
○イ
正解The correct answer is イ, namely 2. Following (1) of the [Guideline for prioritization], calculate for each risk the amount of impact that can be reduced × the probability of occurrence − the cost required for the countermeasure. W gives 500 × 0.3 − 50 = 150 − 50 = 100, X gives 1,000 × 0.2 − 100 = 200 − 100 = 100, Y gives 3,000 × 0.1 − 200 = 300 − 200 = 100, and Z gives 5,000 × 0.1 − 200 = 500 − 200 = 300. The largest value is Z with 300, so Z takes priority 1. The remaining W, X, and Y all tie at 100, so guideline (2) is applied here, giving priority to the one with the smaller cost required for the countermeasure. The costs are 50 for W, 100 for X, and 200 for Y, so the order becomes W → X → Y, which fixes W at 2, X at 3, and Y at 4. What the question asks for is a, which is in the row for W, so the answer is 2 (イ). The key points are to compare not the size of the impact amount itself (Y and Z are larger) but the net effect obtained by subtracting the cost from the expected value, and to apply this two-stage rule in the correct order, using the lower cost as the tie-breaker when values are equal.
×Incorrectウ
3 is X's priority. W, X, and Y all tie at 100 under guideline (1), and guideline (2), which says to give priority to the one with the smaller cost required for the countermeasure, lines them up in the cost order W 50 < X 100 < Y 200, so W becomes 2 and X becomes 3. W's cost is the smallest among the three tied risks, so W never falls behind X into third place.
×Incorrectエ
4 is the priority of Y, which has the largest countermeasure cost among the three tied risks. If one lined them up only by the size of the reducible impact amount, W would appear to come last, but the guideline states that the comparison is not on the impact amount itself but on impact amount × probability of occurrence − cost, and by that measure W is also 100, the same as Y. Since guideline (2) gives priority to the lower cost when values are tied, W actually comes higher (second place), so 4 is wrong.
ここがポイント
When prioritizing risk responses, do not judge by the size of the damage (the impact amount) alone: evaluate the expected value by multiplying it by the probability of occurrence, and then compare the net effect after subtracting the cost of the countermeasure. The value used here is the amount of impact that can be reduced × the probability of occurrence − the cost required for the countermeasure.
When there are multiple evaluation criteria, always rank by the first criterion, and apply the second criterion (here, giving priority to the smaller cost) only to the items that end up tied. Note that if you mix up the order in which the criteria are applied, the rankings get swapped around.