Question

What would most likely occur if an experimental design specifies only a test group

Answer 1

In an experiment, if you don't have a control group (or something to compare your experimental group to) how would you know if what your experiment is showing is normal?

Answer 2

For example, say you're testing the effectiveness of an insulator (something that holds heat in really well).
You would need say, two glass jars with hot water in them and lids on top (preferably with a thermometer sticking through the lid).

One glass jar could be wrapped in idk, fur. That would be your experimental group.

The control group would be just the other glass jar that isn't wrapped in anything. In this case that would show you how much heat would be lost without any other type of insulation.

You could compare your insulated glass results to see how much more heat the one wrapped in fur could hold.

That was not a great example, but I hope it made things a little more clear.