With respect to a control group, you have to remember that it's not sufficient to find just *any* group of people who don't use cell phones. The whole point of a control group is to isolate the variable(s) under study -- cell phone usage in this case, and determine if the variable(s) are correlated with some other measure(s). Otherwise, you can't tell what's responsible for the measured results -- is it the Amish's diet, life style, cell phone usage, etc...
So, the two groups should be similar in all respects, except cell phone usage.
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With respect to a control group, you have to remember that it's not sufficient to find just *any* group of people who don't use cell phones. The whole point of a control group is to isolate the variable(s) under study -- cell phone usage in this case, and determine if the variable(s) are correlated with some other measure(s). Otherwise, you can't tell what's responsible for the measured results -- is it the Amish's diet, life style, cell phone usage, etc... So, the two groups should be similar in all respects, except cell phone usage.