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Can following a vegetarian diet reduce the occurrence of the common cold? An observational study is being planned to compare the average number of colds self-proclaimed vegetarians suffer from each year with the average number of colds non-vegetarians suffer from each year.

Which of the following represents the biggest problem with this type of study?


Blinding is not possible since the individuals in the study will know whether or not they are vegetarian.


It will be impossible to see if there is a relationship between following a vegetarian diet and the occurrence of the common cold.


Confounding could occur. Perhaps vegetarians get fewer colds because they try to follow a healthier overall lifestyle than non-vegetarians.


Undercoverage could occur since individuals who are following other specific types of diets (like gluten free or low fat diets) are not specifically included in the study.


The placebo effect may cause individuals to feel like they do not have a cold when really they do.

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