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Scientists have studied the effects of electromagnetic fields associated with high voltage power lines upon people whose homes are located within 300 yards of the lines. They have compared the growth rate of children who live near the lines with the growth rate of children who live elsewhere. Because they found no significant differences in the growth rates in the two study areas, they have concluded that electromagnetic fields are harmless.
Which one of the following, if true, would most seriously jeopardize the scientists’ conclusion?