What would a control group have added? If you boil water in a beaker, do you need a control beaker where you monitor the temperature but don't light the Bunsen

What would a control group have added? If you boil water in a beaker, do you need a control beaker where you monitor the temperature but don't light the Bunsen burner? Also, how would you run a double-blind test of the foods people are eating? It's pretty hard to disguise steaks ;-) Most nutritional studies are observational, so the numbers are not very reliable. In this case, "subjects were provided with all food for 18 wk, which was prepared and packaged by staff in our research kitchen." That's one reason why the numbers were so small, though it would have been better if they'd been bigger. The most reliable evidence would come from controlling what people ate and what they did. I'd have thought this would be something that could be done with volunteer prisoners.