The most widely used correlation coefficient is the Pearson Product-Moment Correlation (abbreviated “r”), often called the Pearson r.
The Pearson r can compare two sets of linear data (data that changes in a straight line) that are of the ratio or interval type
When determining a correlation or relationship, two scores from each subject must be attained. These scores can not be separated when the data is analyzed. In other words, you must compare Joe’s height with his own weight, you cannot compare Joe’s height with Sally’s weight.