An instructor wishes to find out whether a new teaching method is superior to his usual procedures, so he conducts an experiment. Everyone in his classes is quite excited about the prospect of learning under the new procedure, but he cannot administer the new teaching method to everyone: a random half of the students receive the new method and the remaining half receive the old method. What is the MOST obvious flaw in this experiment?

Placebo effects or experimenter bias are likely to affect results

Explanation:

Due to the fact that everyone in the class is quite excited about being taught by the new method, the most obvious error will be the placebo effect, a phenomenon where some people would perceive some benefit of some sort because they already know what is expected of them. Experimental bias may also occur since everyone is quite excited, but then he chooses to allocate the teaching method to a random half and the old method to the other half. This can cause the students allocated to the old method to provide bad reports and those who are allocated to the new method to claim excitement.