ABSTRACT
Are they cor-related or co-related? Until now we have been considering only single variable distributions, such as height, IQ, etc. and how to describe statistics associated with them. However, two variables (such as depression states and memory) are often inter-related and we wish to discover how, whether one causes the other or whether they tend to be directly or inversely related, etc. In pure sciences, such as chemistry and physics, relations between variables are often perfect or very nearly so. This is rarely the case in the social sciences. Where human behaviour is concerned it would be difficult to think of a case in which one could realistically exclude the operation of all but two variables or even define perfectly the variables over which we have no control.
