In statistics, the degree of association between two random variables .
The correlation between the graphs of two data sets is the degree to which they resemble each other. However, correlation is not the same as causation, and even a very close correlation may be no more than a coincidence. Mathematically, a correlation is expressed by a correlation coefficient that ranges from -1 (never occur together), through 0 (absolutely independent), to 1 (always occur together).