Question: Several studies have documented the susceptibility of people who are HIV-positive to cholera, likely because of weakened immune systems. Researchers in Mozambique (Lucas et al., 2005), a country where an esti mated 20% to 30% of the population is HIV-positive, wondered whether an oral vaccine for cholera would work among people who are HIV-positive. Fourteen thousand people in Mozambique who tested positive for HIV were immunized against cholera. 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 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? State at least one possible confounding variable.