Case Study

Based on the researcher's scenario, explain why they should use Fisher's Least Significant Difference (LSD) test for their post hoc pairwise comparisons instead of running multiple standard tt-tests, and describe what error they are preventing.

Case context: A psychology researcher conducts a study comparing the effectiveness of three different study environments on student concentration. After running a one-way ANOVA, they find a statistically significant overall result. The researcher wants to compare the means of each environment pair to see which ones differ, but they are concerned about the risk of making a Type I error.

Question: Based on the researcher's scenario, explain why they should use Fisher's Least Significant Difference (LSD) test for their post hoc pairwise comparisons instead of running multiple standard tt-tests, and describe what error they are preventing.

Sample answer: The researcher should use Fisher's LSD because it is a modified tt-test procedure designed for post hoc comparisons after a significant one-way ANOVA. If the researcher ran multiple standard tt-tests, the compounded risk of making a Type I error would increase. Fisher's LSD mathematically controls this compounded risk of making a Type I error (mistakenly rejecting a true null hypothesis) while allowing the researcher to evaluate differences between specific pairs of group means.

Key points:

  • Fisher's LSD is designed specifically for post hoc comparisons after a significant one-way ANOVA.
  • Running multiple standard tt-tests increases the compounded risk of making a Type I error.
  • A Type I error is mistakenly rejecting a true null hypothesis.
  • Fisher's LSD mathematically controls this compounded risk.

Rubric:

  1. Explains that Fisher's LSD is suitable because it is designed for post hoc comparisons following a significant one-way ANOVA (3 points). 2. Explains that multiple standard tt-tests increase the compounded risk of making a Type I error (4 points). 3. Correctly defines a Type I error as mistakenly rejecting a true null hypothesis and states that Fisher's LSD mathematically controls this risk (3 points).

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Updated 2026-05-27

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Research Methods in Psychology - 4th American Edition @ KPU

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