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A grid planner studied a sample of substations and found that those with battery storage installed had fewer outages last year than those without. The planner recommends installing batteries at all substations to cut outages. Which of the following would be most useful to determine in evaluating the recommendation?
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Correct answer
D
This is an Evaluate question: the recommendation leaps from a correlation (battery-equipped substations had fewer outages) to a causal claim (batteries cut outages). The key uncertainty is whether the battery-equipped substations differed from the others beforehand. (D) tests exactly that.
If those substations were already in areas with more reliable power lines, their lower outage count may reflect that pre-existing advantage rather than the batteries, and installing batteries everywhere would not deliver the same gain; if they were not already advantaged, the causal claim holds up better. Because the answer swings the evaluation in opposite directions, it is the most useful thing to determine.
(A) concerns financing, which has no bearing on outages. (B) concerns how outages were defined and counted, a measurement choice rather than the causal question. (C) concerns a long-run technology trend, not the comparison within this sample. (E) names customer type, which by itself does not isolate the causal effect of the batteries.
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