A common approach adopted in the old days was matching. As it turned out, however, matching wasn't all that it was cracked up to be. The biggest problem was that it simply didn't work. Some researchers tried matching their participants on several dimensions at the same time, but this often proved to be very difficult. The solution to this problem turned out to be random assignment.
Random assignment is the closet thing to magic that experimentalists have discovered.
Whereas matching involves placing people in experimental conditions on a very systematic basis, random assignment involves the placement of people in groups on a totally arbitrary basis. Technically, this means that every participant has an equal chance of being assigned to any particular condition of an experiment. For example, an experimenter might assign people to conditions in a study by either flipping a coin or drawing names from a hat. And random assignment, like coin flips, is fair and logical. Besides being a lot easier and quicker than matching, random assignment does what matching cannot.