Answer:
Age, gender, culture of candidates.
If the research was performed in mice, living conditions should be controlled.
Dependent variables: Hygiene level.
Explanation:
First of all, the study in human beings should consider about differences in ages, genders, and cultures. Because in different ages, people have different expectation of hygiene. It is similar for gender in which female is probably more hygiene than male. There is not any specific place or country in which this study would be held. However, in general, many cultures define differently the hygiene, such as Western Asia, Eastern Asia, European, and so on.
Second of all, if they are doing the experiments on mice, they should consider about living conditions. These are the main variables, which impact on hygiene levels.
Lastly, because it is the study about a hygiene hypothesis. Thus, the hygiene levels should be consider.