'''Correlation''' refers to how a characteristic is common to a group, as in the '''correlation''' between hard work and success. The correlation is usually not 100%, as a few people may succeed without hard work, and a few people may fail with hard work. But there is a correlation if typically hard work does result in success.
The '''correlation coefficient''', also known as '''Pearson's r''', is a statistical measure of association between two ratio variables. It is defined as: