In their quest to discover what is caused by what, scientists frequently use a basic experimental design. The logic of the design is very simple, but doing it is sometimes very difficult. It begins with an hypothesis, which is a statement that factor X is one of the causes of event Y. To find out if the statement is true, a scientist conducts an experiment in which there are two conditions. One condition includes factor X, the other does not. If event Y occurs only in the condition including factor X, one can conclude that the hypothesis is true, that factor X does indeed cause event Y.
The difficulty in using the experimental method is in insuring that factor X is the only difference between the two conditions. Let me illustrate the method and the difficulty with a common question by students: Should one study with background music playing? We first state the question as an hypothesis: Background music is beneficial to learning. To test that hypothesis, we need to have students study with music and without music, and then somehow measure how much they learned. Conceptually, the design is clear; practically, it is very difficult.
First we must be sure that the material being learned with music is equally difficult as that being learned without music. Perhaps the hypothesis is true for some kinds of material and not for others, and so we probably should include several kinds. Perhaps the effects depends on the type of music, so we should also include several types of music. The result may depend on whether the student usually has music playing while studying, so we will need to take past experience into consideration. For that matter, some students may be biased in such a way that they work harder with music in order to prove that it is beneficial. Then there are many other factors to be balanced: the time of day, state of hunger, importance of the material, etc.
The fact that it may be difficult to satisfy the everything-else equal requirement of the experimental method should not dissuade one from trying to use it. It is the best way to avoid superstitious beliefs. For example, you may have seen a commercial that some cereal will improve your performance. You may then try the cereal and, by chance, do better than usual. Should this happen more than once, you may be firmly convinced that the commercial was true. Perhaps it is true but as a scientist, you would want to compare several cereals under nearly-identical testing conditions. You can learn a lot about yourself and the world around you by adopting the scientific method.