Data Collectionmedium
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A school district develops a new reading intervention for struggling readers. They administer a reading comprehension test (scale: 0–100 points) before and after a 12-week intervention to 240 students.

Results:

  • Students who scored below 30 at baseline (n = 160): improved by an average of 16 points
  • Students who scored 70–100 at baseline (n = 80): improved by an average of 3 points

The program director concludes: 'The intervention is much more effective for lower-performing students.' Which alternative explanation is most plausible?