What is control group?

A control group is the baseline a study compares its treatment group against.

If the people drinking the energy juice knew they were drinking an "energy" juice and the water drinkers knew they were not, the comparison is broken: expectations, not juice, could explain the difference.

Good LSAT answers attack studies by showing the control group differed from the treatment group in some other relevant way.

No control group at all means there is nothing to compare the result to. The study proves nothing.