How do you answer questions that ask you to evaluate the significance of the difference?

For example:

"Two samples of 21 men each are brought into a room and told they are to help another person in the next room learn a simple problem by giving him an electric shock as a punishment for wrong responses. The subject has a dial with 20 levels of shock that he can administer, 20 being the highest level.Another person is with the subject during the observation; he encourages one group to administer higher and higher levels of shock, whereas he does not so encourage members of the other group. The person supposedly receiving the shocks and the "persuader" are really actors, but the people administering the "shocks" do not know this. The point of the study is to determine whether people are influenced by pressure from others when given the opportunity to hurt other individuals without fear of disapproval. The hypothetical results are these:

The average shock level administered by the subjects who were encouraged to use higher levels was 16.2 (s12 = 12), and that administered by the other group was 13.8 (s22 = 13). Evaluate the significance of this difference."