A graph question about the main source of a change tests whether the numbers support the full claim. Read the claim, then the graph’s title, axes, and key so you know which groups and times to compare. Work out each group’s change, so you can see how much each rose or fell. Compare the sizes of the changes to test where most of the overall change came from. A choice can report true numbers without showing which group contributed most.
Hints
- Hint 1
Look at the manager’s claim about the source of the reduction. What must the data show about the different defect types?
- Hint 2
Subtract each after count from its before count. How much does each defect type contribute to the overall decrease?
Step-by-step
Compare contributions to the decrease
Step 1Find both parts of the claim
The manager says the reduction came primarily from fewer wheel-alignment defects, not from comparable improvements across all defect types. What would show both parts? We need wheel alignment to account for most of the decrease, with much smaller changes in the other types. A large starting count by itself won’t tell us how much a category fell.
- Step 2
Read the paired monthly counts
The title and axes show average monthly defect counts, with one pair of bars for each defect type. These are monthly averages, not six-month totals. Which bars tell us how much a type changed? The key marks light gray as before recalibration and dark gray as after, so we compare the pair within a category. Wheel alignment went from 64 to 18, a monthly decrease of 46 defects.
- Step 3
Measure the rest of the decrease
How much did the other defect types contribute? Paint finish fell by 3 per month, brakes fell by 2, and packaging rose by 1. Their combined counts went from 28 + 22 + 16 = 66 to 25 + 20 + 17 = 62, a net decrease of only 4 defects. Wheel alignment’s decrease of 46 is much larger, so it accounts for most of the overall reduction in defects. The choice comparing 64 to 18 with 66 to 62 gives that contrast. Choice A.