Researchers have noted a decline in cognitive functioning as people age (Bartus, 1990). However, the results from other research suggest that the antioxidants in foods such as blueberries can reduce and even reverse these age-related declines, at least in laboratory rats (Joseph et al., 1999). Based on these results, one might theorize that the same antioxidants might also benefit elderly humans.
Suppose a researcher is interested in testing this theory.
The researcher obtains a sample of n = 16 adults who are older than 65, and gives each participant a daily dose of a blueberry supplement that is very high in antioxidants. After taking the supplement for 6 months, the participants are given a standardized cognitive skills test and produce a mean score of
M = 50.2. For the general population of elderly adults, scores on the test average µ = 45 and form a normal distribution with s = 9.
a. Can the researcher conclude that the supplement has a significant effect on cognitive skill? Use a two-tailed test with a = .05.
b. Compute Cohen's d for this study.
c. Write a sentence demonstrating how the outcome of the hypothesis test and the measure of effect size would appear in a research report.