Читать книгу Applied Biostatistics for the Health Sciences - Richard J. Rossi - Страница 29

Determine whether it would be possible to perform an experiment in each of the scenarios given below.

1 a. A nutritionist is interested in comparing several different diets in a prospective study. The treatments that will be compared are 10% fat in the diet, 15% fat in the diet, and 25% fat in the diet.

2 b. A pediatrician is interested in studying the effects of a mother’s use of tobacco on the birth weight of her baby. The two treatments that are to be compared are smoking during pregnancy and not smoking during pregnancy.

3 c. A medical researcher is studying the efficacy of vitamin C as a preventive measure against the common cold. The two treatments that are being compared are 1000 mg vitamin C and 1000 mg placebo.

Solutions

1 a. Because the researcher can assign the subjects to each of the three diets in this study, it could be performed as an experiment.

2 b. Because smoking is known to have harmful effects on a fetus, it would be unethical for a pediatrician to assign some mothers smoke during pregnancy and others to not smoke during pregnancy. This study would have to be performed as an observational study by comparing the weights of babies born to mothers who chose to smoke during pregnancy with babies born to mothers who did not smoke during pregnancy.

3 c. Because the medical researcher could assign the subjects to these two treatments, it could be performed as an experiment.

An important advantage experiments have over observational studies is that it is possible in an experiment to control for external factors that might cause differences between the units of the target population. By controlling for the external factors in an experiment, it is possible to make the groups of units assigned to different treatments (i.e., treatment groups) as alike as possible before the treatments are applied. Moreover, in a well-designed experiment when the value of an explanatory variable is changed while no other changes take place in the experimental conditions, any differences in the responses are most likely due to the change in the value of this explanatory variable.

On the other hand, it is much harder to control external factors in an observational study because the units come to the researcher already assigned to the treatments, and thus, in an observational study there is no guarantee that the treatment groups were alike before the treatments were assigned to the units. Because experiments can be designed to control external factors, they can be used to establish evidence of causal relationships; an observational study generally cannot provide strong evidence of a causal relationship because uncontrolled external factors cannot be ruled out as the potential cause of the results.