1. An event that does not constrain or determine another so that the latter cannot be said to be a function of the former, but is closely associated with it, may be in the relationship of being mutually determined by a third event.
2. Mathematically, the relationship between variable measures, where increases or decreases tend to be parallel or proportional.
3. Positive correlation: change in the same direction between variables.
4. Negative correlation: change in opposite directions between variables.
5. Linear correlation: change maintained in a certain proportion.
6. Non-linear correlation: variable rates of change.
7. Correlation theory assumes that phenomena obey probabilistic laws. Given one random event a second random event can be expressed probabilistically in terms of its correlation.