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Based on the properties of the chosen statistical test, what must the researcher decide regarding the null hypothesis, and why? Explain this outcome in terms of the test's critical value.
Case context: A developmental psychologist hypothesizes that a new interactive reading program will improve vocabulary scores in first graders compared to a standard curriculum. Before collecting any data, the researcher decides to use a one-tailed test because they are only interested in whether the program increases scores. After running the study, the data shows that the students in the new reading program actually have substantially lower vocabulary scores than the standard group.
Question: Based on the properties of the chosen statistical test, what must the researcher decide regarding the null hypothesis, and why? Explain this outcome in terms of the test's critical value.
Sample answer: The researcher must fail to reject the null hypothesis, even though the lower scores appear substantial. Because they specified a one-tailed test prior to data collection, they utilized only a single critical value placed in the expected (positive) direction. Therefore, this test completely eliminates the possibility of rejecting the null hypothesis when the results go in the opposite, unexpected direction.
Key points:
- The researcher cannot reject the null hypothesis.
- The one-tailed test was decided prior to data collection to look only for an increase.
- The test utilizes only a single critical value located in the expected direction.
- The test completely eliminates the possibility of rejecting the null hypothesis if results go in the opposite direction.
Rubric: The response should diagnose that the researcher cannot reject the null hypothesis and justify this by explaining that a one-tailed test uses a single critical value for the expected direction, which entirely eliminates the possibility of detecting significance in the opposite direction.
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Research Methods in Psychology - 4th American Edition @ KPU
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