In chapter 6, you learned that a sensitive measure should
  1. produce a wide range of scores (so that the measure may capture the wide range of differences in participants)
  2. and
  3. do so reliably (so the different scores are not merely due to random error).

The short simulation below will help you understand how random error can interfere with sensitivity.
As you move the green slider, the slider reflects the participants "true" score, whereas the ball reflects what score the insensitive measure would give the participant.

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