The authors of the paper "Inadequate Physician Knowledge of the Effects of Diet on Blood Lipids and Lipoproteins" (Nutrition Journal [2003]: 19-26) summarize the responses to a questionnaire on basic knowledge of nutrition that was mailed to 6000 physicians selected at random from a list of physicians licensed in the United States. Sixteen percent of those who received the questionnaire completed and returned it. The authors report that 26 of 120 cardiologists and 222 of 419 internists did not know that carbohydrate was the diet component most likely to raise triglycerides.

a. Estimate the difference between the proportion of cardiologists and the proportion of internists who did not know that carbohydrate was the diet component most likely to raise triglycerides using a 95% confidence interval.

b. What potential source of bias might limit your ability to generalize the estimate from Part (a) to the populations of all cardiologists and all internists?