Unit 15: Experimental Design Revisited

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First, read this article from the New England Journal of Medicine (in pdf format.) To give you some context, here's what ABC news said on May 2003:

Controversial diet maven Robert Atkins may no longer be around, but a tug-of-war still lingers over his belief the road to weight loss is paved with bacon cheeseburgers — hold the buns. Now, studies may help resolve the dispute over the effectiveness of the so-called Atkins diet, which advocates low carbohydrate and high fat intake. Popular among dieters but disputed by their doctors, the Atkins diet has remained a huge question mark in the quest for reliable weight loss. Many experts have remained critical of the approach, but with little evidence to back up their critique. The new research, published in the New England Journal of Medicine, compares the weight loss of severely obese individuals eating according to Atkins with those eating according to a conventional low-fat, low-calorie nutrition plan.

Now answer these questions. Put your answers in the form of a report, and email to your instructor. Call the file ms15yourlastname.

  1. What was the objective of this study? What variables were measured to attain this objective? Identify all the variables in the study and classify as categorical or quantitative.
  2. Table 1 gives some baseline data on the participants. The researchers wrote in the footnote of the table there were no significant differences between the two groups. Test if the researchers’ statement is correct for the gender breakdown in the two diet groups. Check the assumptions, before a test. Write your conclusion.
  3. Table 1 gives the standard deviations of Body-mass index, weight, and blood pressure. How useful is this information? Can it tell you whether the data are approximately normal? What information would be more useful?
  4. In Table 2, there is a column with p-values. What test was performed to find the p-value? You don’t have to perform the test. However, state the null hypothesis, the alternative. What is your interpretation of the p-value for the 12 month difference in percentage weight loss?
  5. At the bottom of Table 2, there are two footnotes regarding p-values. Use this information to answer the following question: Did any of the two diets showed a statistically significant weight loss for the participants?
  6. On page 2085 the authors mentioned attrition (subjects who had dropped out from the study). Why did they bothered to show Table 2 and Table 3?
  7. In the discussion on page 2089 you find the statement “as compared with conventional diet, the low-carb diet was associated with a greater improvement in some risk factors for coronary heart disease." How does Figure 3 support this statement?
  8. In the comment section Dr. Aziz is related to the issue of statistical significant and clinically significant differences. Explain the difference between the two.

You might be interested to read some of the letters to the editor regarding this paper. If you have any comments on these, please include them in your milestone.