The following questions refer to Example 7.9 and Section 7.7.8. a. Evaluate the parallelism assumption in mod_ancova. What do you conclude?

b. Compute the adjusted difference in mean weight gain using mod_ancova. Verify that you obtain the same value as the parameter estimate when using sample means.

Data from Example 7.9

Data from Section 7.7.8

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Example 7.9 (Does where you live affect your weight?). The NHANES I Epidemiologic Follow-up Study (NHEFS) [CDC, 2021] is a national longitudinal study that was jointly initiated by the National Center for Health Statistics and the National Institute on Aging in collaboration with other agencies of the United States Public Health Service. The NHEFS was designed to investigate the relationships between clinical, nutritional, and behavioral factors assessed in the first National Health and Nutrition Examination Survey NHANES I and subsequent morbidity, mortality, and hospital utilization, as well as changes in risk factors, functional limitation, and institutionalization. The NHEFS cohort includes all persons 25-74 years of age who completed a medical examination at NHANES I in 1971-75 (n = 14,407). It is comprised of a series of follow-up studies, four of which have been conducted to date. The first wave of data collection was conducted for all members of the NHEFS cohort from 1982 through 1984. Continued follow-up of the NHEFS population was conducted in 1986, 1987, and 1992 using the same design and data collection procedures developed in the 1982-1984 NHEFS, with the exception that a 30-minute computer-assisted telephone interview was administered rather than a personal interview, and no physical measurements were taken. What is the effect of living in a city compared to city suburbs on weight gain? The first and fourth waves of the NHEFS provide an opportunity to design an