Part II (15 marks)

HIV vaccine trials

An important early step in drug development is to evaluate the safety of newly developed drugs. In one such trial for HIV vaccines currently underway in Trinidad, Haiti and Brazil, researchers are comparing the effects on the immune system of a new vaccine called ALVAC, to a previously investigated vaccine called HIV-1 SF-2. They will administer one of the two vaccines to each participant in the study, and then monitor their immune response. For simplicity, suppose they are measuring the levels of a specific antibody.

1.

(1 mark) What is the response variable?
Solution: antibody levels

2.

(2 marks) What is the factor in this experiment and its two levels?
Solution: The factor is vaccine and its levels are ALVAC and HIV-1 SF-2.

3.

(6 marks) Suppose you have 50 volunteers to take part in the study. Sketch an appropriate study design, including the numbers assigned to each treatment group.
Solution:

           /-- Group 2          Treatment 1 ---\
Random    /   25 subjects ----> HIV-1 SF-2      \  Treatment
Allocation\                                     /  Comparison
           \-- Group 3          Treatment 2 ---/
              25 subjects ----> ALVAC

4.

(4 marks) Use the following line from a table of random numbers to select the first four subjects to recieve the new treatment.

19723 96039 05756 28713 96409

Solution: 19, 39, 5, 13

5.

(3 marks) The following pair of boxplots show hypothetical results on the antibody measurements in the two groups.

Which group has a larger inter-quartile range?
Solution: ALVAC
Which group has a larger median antibody measurement?
Solution: SF-2
Which group has a larger minimum antibody measurement?
Solution: SF-2