Question: Cancer, Weight Loss, and Protein Turnover

Data File: Cancer Protein Turnover

Weight loss among cancer patients is a well-known phenomenon. Of interest to clinicians is the role played in the process by metabolic abnormalities. One investigation into the relationships among these variables yielded data on whole-body protein turnover (Y) and percentage of ideal body weight for height (X). Subjects were lung cancer patients and health controls of the same age.

1. Perform and summarize your findings from an appropriate t-test (pooled or non-pooled) to compare the mean protein turnover of lung cancer patients to that for healthy controls. Quantify the size of the difference in these means using a 95% CI for the difference in the population means. Interpret this interval. (3 pts.)
2. Construct a scatter plot of these data using a different plotting symbol for each of the two groups. Fit a separate regression line for each group (use the Group By… option from the Bivariate Fit pull-down menu). Discuss any differences and similarities between these two regression lines. (3 pts.)
3. Fit a model that incorporates both ideal body weight for height and cancer status. Be sure to allow for both the intercepts and slopes to be different (i.e. fit the unrelated lines model to these data). Is the interaction term needed? Explain (2 pts.)
4. Fit the parallel lines model to these data and interpret the parameter estimates. May one conclude that the two sampled populations differ with respect to mean protein turnover when percentage of ideal weight is taken into account? (3 pts.)
5. Give and interpret a 95% CI for the difference between lung cancer patients and healthy controls adjusting for the percentage of ideal body weight. How does this interval compare with the confidence interval you found in part (a)? (3 pts.)
6. Check model assumptions for these data. Discuss your findings. (3 pts.)

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