Answers to Exercises

Summary

Chapter 1 Observational Studies and Experiments

Exercise Set A

1. In table 1, there were 837 deaths from other causes in the total treatment group (screened plus refused) and 879 in the control group. Not much different.

Comments. (i) Groups are the same size, so we can look at numbers or rates. (ii) The difference in number of deaths is relatively small, and not statistically significant.

1. This comparison is biased. The control group includes women who would have accepted screening if they had been asked, and are therefore comparable to women in the screening group. But the control group also includes women who would have refused screening. The latter are poorer, less well educated, less at risk from breast cancer. (A comparison that includes only the subjects who follow the investigators' treatment plans is called “per protocol analysis,” and is generally biased.)

2. Natural experiment. The fact that the Lambeth Company moved its pipe (i) sets up the comparison with Southwark & Vauxhall (table 2) and (ii) makes it harder to explain the difference in death rates between the Lambeth customers and the Southwark & Vauxhall customers on the basis of some difference between the two groups–other than the water. For instance, people were generally not choosing between the two water companies on the basis of how the water tasted. If they had been, self selection and confounding would be bigger issues.[…]