Introduction

With so many different methods nowadays in psychology, it is not possible to be acquainted with all of psychology today. But when we look at the simpler times of psychology, we have two historic streams of method, which are widely used since the last century of our science: experimental psychology and correlational psychology. With both of these focussing on different things, the convergence of these two streams is still in the making.

The separation of the disciplines

Experimental psychology is when the scientist makes changes to the conditions to observe their consequences, and is the more coherent of the two disciplines. Correlational psychology also qualifies as a discipline because it asks a distinctive type of question and has technical methods to examine the question and the data. The correlator focuses more on the already existing variation between individuals, social groups, and species. Instead of the variation he himself creates, as the experimental psychologist does. With the experimental method it is a virtue that the variables are controlled, so it permits rigorous tests. While the correlator observes and organizes the data that nature has created. 

Characterization of the disciplines

Experimental psychology

In the beginning, experimental psychology was a replacement for naturalistic observation. With standardization of tasks and conditions they could get reproducible descriptions. Later the focus shifted to the single manipulated variable, and after that to multivariate manipulation. Another great development has been its concern with formal theory. But a problem that we do have in these experiments is due to individual variation. Because of this variation we get 'error variance'. We can reduce this error variance by selecting certain properties.

Correlational psychology

On the other hand, the correlational psychologist sees individual and group variation as important effects of biological and social causes. He wants to see what characteristics determine its mode and degree of adaptation, and his goal is to predict variation within a treatment.

The shape of a united discipline

It is not sufficient for each discipline to borrow from the other. A united discipline will study both experimental psychology and correlational psychology, but will also concern itself with the interactions between organismic and treatment variables. We should invent constructs and form a network laws which permits prediction. Investigators can use different methods, but still test the same theoretical propositions.

There already have been methodologies proposed for a joint discipline, one of the many choices is analytic procedures. Eventually, these two disciplines will become one, with a common theory, a common, method, and common recommendations for social betterment. A whole new dimension will be discovered and we will come to realize that organism and treatment are inseparable.