To study the effectiveness of vitamin C in preventing colds, a researcher recruited 200 volunteers. She randomly assigned 100 of them to take vitamin C for 10 weeks and the remaining 100 to take nothing. The 200 participants recorded how many colds they had during the 10 weeks. The two groups were compared, and the researcher announced that taking vitamin C reduces the frequency of colds.

The claim made by the researcher lacks the validity on basis of reasons as indicated in the explanation.

Step-by-step explanation:

The claim made by the researcher is not valid on basis of following reasons:

Interacting variables are not mapped accurately. The additional variables as the health status of the volunteers, eating and fitness habits are not considered in the consideration. This results in a in complete validation. The placebo effect is not catered accordingly. As the half of the volunteers are taking nothing, this is not the direct influence of the placebo and thus double-blinded testing for placebo effect is to be ensured in future experiments. The environmental effects are not considered as the effect under study is dependent on the environmental effects, thus the claim is biased and the generalization is not valid.