Researchers conducted a study to see the effect of specific lifestyle and dietary changes for preventing long-term weight gain. The study involved the consolidation of three cohorts from the Nurses' Health Study (NHS): (1) cohort of 121,701 female registered nurses, who enrolled in 1976; (2) the Nurses' Health Study II (NSH II), a cohort of 116,686 younger female nurses, who enrolled in 1989; and (3) the Health Professionals Follow-up Study (HPFS), a cohort of 51,529 male health professionals, who enrolled in 1986. Participants were followed with biennial questionnaires concerning medical history, lifestyle, and health practices. The baseline year of the study was used to determine diet, physical activity, and smoking habits of the participants. The final analysis included 50,422 from the NHS, 49,898 from the NHS II, and 22,557 from the HPFS (those with health issues or obesity were excluded from the study). Individuals were categorized based on various lifestyle choices. For example, those who disclosed an increase in consumption of french fries had a mean weight gain of 3.35 pounds over a 4-year period with a 95% confidence interval of 2.28 pounds to 4.42 pounds. Among those who disclosed a change in smoking status from former smoker to current smoker the mean weight gain was -2.47 pounds over a 4-year period with a 95% confidence interval of -3.82 pounds to -1.12 pounds. Source: Dariush Mozaffarian, M.D. et al. "Changes in Diet and Lifestyle and Long-Term Weight Gain in Women and Men," The New England Journal of Medicine 364;25
(a) Explain what it means for this study to be a cohort study.
(b) Is the variable weight gain quantitative or qualitative? How was the variable measured?
(c) Within the cohort "individuals who increased their consumption of french fries during the observation period," there was a mean weight gain of 3.35 pounds over the 4-year period. Is the variable "increase in consumption of french fries" quantitative or qualitative? Explain.
(d) What is the margin of error for the 95% confidence interval for increase in weight gain among those who increased their consumption of french fries?
(e) What does a mean weight gain of -2.47 pounds among those whose smoking status changed from former to current smoker suggest?
(f) Interpret the 95% confidence interval for the mean weight gain among those whose smoking status changed from former to current smoker.
(g) Describe the population to which the results of this study apply?