You conduct a study in which you compare the blood pressure of three groups of p

Question

You conduct a study in which you compare the blood pressure of three groups of people diagnosed with hypertension, one group taking a standard blood pressure medication, one group taking a new treatment, and a third group assigned to a strict diet and exercise protocol. The patients were allowed to choose which treatment they wanted to try. You measure the patients' blood pressure 6 months after the patients began their treatment. Which would be the best null hypothesis to test using ANCOVA in this study?

A. There is no difference in the mean blood pressure 6 months posttreatment in the three groups after adjusting for pretreatment blood pressure and other potential confounders.

B. There is no difference in the probability of having hypertension 6 months after treatment in the three groups.

C. There is no difference in the mean blood pressure at 6 months posttreatment in the three groups.

D. There is a small difference in the mean blood pressure 6 months posttreatment in the three groups after adjusting for pretreatment blood pressure and other potential confounders.

E. None of the above

Explanation / Answer

A option.

The null hypothesis and the alternative hypothesis for ANCOVA are similar to those for ANOVA. Conceptually, however, these population means have been adjusted for the covariate. Thus, in reality, the null hypothesis of ANCOVA is of no difference among the adjusted population means.

If the relationship is nonlinear, the adjustments made in the ANCOVA will be biased; the magnitude of this bias depends on the degree of departure from linearity, especially when there are substantial differences between the groups on the covariate. Thus it is important for the researcher, in preliminary analyses, to investigate the nature of the relationship between the dependent variable and the covariate (by looking at a scatter plot of the data points), in addition to conducting an ANOVA on the covariate

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