Data InsightsMulti-Source Reasoning

Free GMAT Multi-Source Reasoning Practice Question

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Glaze Works fires ceramic tiles in batches. Each batch gets a defect score (percentage of tiles with glaze flaws); lower is better. After one quarter the plant manager singled out 10 batches and assigned them a new slow-cool firing tweak for their next run. She reports: those 10 batches averaged a 22 percent defect rate before the tweak and only 13 percent after, so the slow-cool tweak cut defects by nine points. We should roll it out plant-wide.

Two comparison groups of batches were also tracked across the same two runs. Their mean defect rates are below. (The identity of each group and how it was formed are given in the Methodology note.)

Batch group	Mean defect rate first run	Mean defect rate second run
Comparison group 1	12 percent	12 percent
Comparison group 2	12 percent	11 percent

A reviewer is checking whether the data establish that the slow-cool tweak caused the improvement in the flagged batches. For each statement, select Yes if it is accurate given the sources; otherwise, select No.

Statement 1: The flagged 10 were selected at random from the quarter's batches. Statement 2: Because the flagged batches were chosen for having the worst first-run scores, some improvement on their second run would be expected even without any tweak. Statement 3: The flagged batches' 22-to-13 improvement, by itself, proves the tweak lowered defects.

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Answer & Explanation

Correct answer

1: No · 2: Yes · 3: No

Statement 1: the Methodology note says the 10 batches were flagged because they scored worst on their first run, the opposite of random. No.

Statement 2: the Study summary reports the flagged batches moved from 22 percent to 13 percent, and the Methodology note says they were selected for the extreme low end of a measure that varies run to run even with no method change; a group chosen for an unusually bad score tends to move back toward the average on re-measurement (regression to the mean), so some of the 22-to-13 improvement is expected with no tweak. You cannot reach this conclusion from the numbers alone, because the regression trigger is the selection rule, which lives only in the Methodology note. Yes.

Statement 3: the drop does not by itself prove the tweak worked, because regression predicts a drop anyway, and the no-tweak group identified in the Methodology note moved only 12 to 11 in the Comparison data, showing the population is stable on its own. No.