An inference question about competing causes tests what timing and group comparisons can show. Read the words leading into the blank, then name the possible causes and the result to explain. Ask what came first, because a cause must come before the effect it is supposed to explain. Then compare groups: an effect appearing without a possible cause weakens that explanation. Your conclusion should weigh the evidence without claiming it proves more than it does.
Hints
- Hint 1
Read the monthly records and put the two policy changes and the sharpest rise in time order.
- Hint 2
Look at the branches that did not change their hours. What does comparing their checkout results with the other branches let you test?
Step-by-step
Test the two causes
Step 1Name the result and possible causes
The administrators credited longer evening hours for a 30% rise in checkouts that year. What result are we explaining? We're testing why checkouts rose by comparing the two possible causes: ending overdue fines and extending evening hours. The administrators' view needs support from the records.
- Step 2
Use the order of events
Could the July change have started the sharpest rise? The rise began in March, immediately after the library ended overdue fines, while evening hours were extended in July. A cause has to come before its effect, so the hour extension can't explain why the rise began. It might have helped later, but that doesn't explain the March start.
- Step 3
Compare branches with different hours
What do the branches tell us that the dates alone don't? Branches that did not alter their hours had checkout increases similar to those that did. The rise appeared both with and without longer hours, so the branch comparison weakens the hours explanation. An hour extension wasn't needed for branches to show a similar gain.
- Step 4
Make the smallest supported conclusion
How far can we take the conclusion? Taken together, these observations suggest that calls for a supported explanation, not proof of the only possible cause. We can favor fine elimination over the hour extension, so the choice's more likely responsible gives us the smallest supported conclusion. Choice D.