We must be careful to remember that correlation does not assume causation. For example, there is a low positive correlation between the rate at which ice cream melts on the sidewalks of New York and the number of deaths in Bombay, India. But one does not cause the other. Both are affected by a third factor, that is, the increase in temperature during summer months. Correlation may lead us to search for a common cause but it does not prove that a cause and effect relationship exists.
Much psychological information comes from the administration of tests.
Two criteria that a test must meet are reliability and validity. These are both measured by correlations. A test is reliable when repeated administrations to the same subjects have a high positive correlation. Reliability, then, is a measure of the degree to which repeated measurements of the same subject give the same reading. If they are, then we could say that the test is reliable.