With animal models to work with, Marshall could have gathered many more test subjects and divided them into test groups to gather more reliable data. He could have also established a test group had there been an animal model to work with.
Work Step by Step
Marshall had issues infecting animals with the H.pylori bacteria, and since human testing was unethical, he was only able to gather data from himself and another volunteer. This created plausible, but highly variable data that would have had issues had there been conflicting results following data collection (e.g. Marshall showed symptoms but the volunteer did not). The lack of human test subjects also meant that there was no control group to work with, and that subjects could not be divided into test groups.