Answer:
The variables that must be controlled are:
1. Age
2. Vaccination
3. Diet
4. Environmental Temperature
5. Clothing
6. Body weight
7. Drugs
Explanation:
The type of experiment described in this study is called case control experiment. It essentially involves a kind of experiment where a substance or health intervention ( in this case, Vitamin D) is assigned to a group of participants and they are observed for the development or not of a particular health outcome ( in this case, cold) and the results are compared with another group of individuals of the same category that did not have access to the substance or health outcome. Since most health outcomes are not caused by only one factor, it is important to control other factors that could cause the same effect as the substance administered. For example, if a study is to be done on changes of skin colour, associated with a particular cream, we know that staying in the sun for a long time can cause changes in skin colour or toning, therefore the participants are not exposed to sunlight as much as possible during the period of the experiment. sunlight is a variable that will be controlled to ensure that the result is reliable.
Now back to this experiment, let me explain the factors i listed and how the reason for the choices
1. Age: children and aged people are more prone to cold than adults therefore for the experiment, the age bracket should be considered. there should not be a mix of both children and adult or aged people.
2. Vaccination: cold is caused commonly by viruses example is the Rhinovirus. hence if some of the participants are immunized against these viruses, it will serve as an additional protection against cold creating a bias against those who have not been vaccinated. So all the individuals can be vaccinated or not vaccinated.
3. Diet: what i had in mind in this case is mostly drinks that help protect against cold like hot beverages, tea or coffee. The ingestion of these diets have to be controlled and unified in all the individuals within the experiment groups.
4. Environmental Temperature: The participants should as much as possible be exposed to the same kind of experimental temperature because it is a very high determinant of whether a person gets a cold or not.
5. Clothing: The same kind or categories of clothing should be worn by the participants of the experiment within the time of the experiment, to ensure that the bodies of the participants are exposed to the same degree to the environment.
6. Body Weight: Obese individuals have a high level of subcutaneous fat, which can protect against development of cold. Hence the individuals to be recruited should approximately have the same body weight.
7. Drugs: within the period other drugs that helps to protect against developing a cold should be avoided by the individuals.
There could be more factors that need to be controlled, you can them out but if the ones I have listed are basically controlled, the results will be reliable.
