Answer
Yes, that could bias the results because the NFIP investigators associated what they saw with they knew about the subjects. Thus, if the NFIP investigators had not known if the children who contracted polio had gone swimming, the study would be less biased.
Work Step by Step
1. First, determine whether the subjects are aware of their grouping.
2. Then, determine whether the investigators are aware of the subjects' grouping.
3. If either of the above is true, then the study is likely to be biased.
