Final answer:
The difference between the control and experimental group in the study, with an effect size index of 0.55, is described as a medium effect size according to Cohen's standards.
Step-by-step explanation:
In the study, the difference between the control group who walked half a mile per day, and the experimental group, who walked further, is represented by an effect size index, d, of 0.55. According to Cohen's standards on effect sizes, a d value of 0.55 should be described as a medium effect size, since it is between Cohen's threshold of 0.5 for medium and 0.8 for large effect sizes. This indicates that the magnitude of the difference in walking distances between the two groups is noticeable and not trivial, though not as strong as it could be with a larger effect size.
Generally, an effect size of this magnitude suggests that the intervention had a moderate impact on the outcome, which in this case is the amount of walking done by the experimental group compared to the control group. In practical terms, while there is a significant difference between the groups, it is not a very large difference. Researchers and practitioners would take this into account when interpreting the results of the study or considering similar interventions.