Several studies have documented that people who are HIV-positive are susceptible to cholera (likely because of having a weakened immune system). Researchers in Mozambique (Lucas et al., 2005), a country where an estimated 20% to 30% of the population is HIV-positive, wondered whether an oral vaccine for cholera would work among people who are HIV-positive. Cholera immunization was administered to 14,000 people in Mozambique who tested positive for HIV. Soon thereafter, an epidemic of cholera spread through the region, giving the researchers an opportunity to test their hypothesis. 

a. Describe a way in which the researchers could have conducted an experiment to examine the effectiveness of the cholera vaccine among people who are HIV-positive. 

b. If the researchers did conduct an experiment, would this have been a between-groups or a within-groups experiment? Explain. 

c. The researchers did not randomly assign participants to vaccine or no-vaccine conditions; rather, they conducted a general mass immunization. Why does this limit their ability to draw causal conclusions? Include at least one possible confounding variable. 

d. The researchers did not use random assignment when conducting this study. List at least one practical reason and at least one ethical reason that they might not have used random assignment.