A friend states that since he has been taking vitamin C, he has not had a single cold. His observation has little value in assessing the effects of vitamin C because

A friend states that since he has been taking vitamin C, he has not had a single cold. His observation has little value in assessing the effects of vitamin C because

a. there was no control group for comparison.
b. he has not calculated a correlation coefficient.
c. of the effect of the observer on the observed.
d. he does not specify his dosage of vitamin C.
He needed a person who hadn't been taking vitamin C (the control group) to test of the vitamin C had any contribution.
He did not need to test the dependence of the vitamin C for the cold therefore he did not need to calculate a correlation coefficient.
The observer wouldn't make a difference on the effects.
The dosage of vitamin C is not important without knowing that vitamin C has any effect at all.


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