Question

In a medical study involving one hundred pairs of identical twins, one member of each pair tests positive for a particular disease, while the other member of each pair does not. One researcher noted that the average body temperature of those who have the disease is 98.0°, and the average body temperature of those who do not have the disease is 98.3°.

The researcher then concludes, "People who have this particular disease have a lower body temperature." Why is this probably not a valid claim?