Correlation is the study of the liner relationship between two variable when there is a relationship of quantitative measures between two set of variable the appropriate statistical tool for measuring the relationship and expressing each in a precise way is known as correlation. For example there is a relationship between the heights and weights of persons demand and prices of commodities etc.
According to W.I. King Correlation means that between two series or groups of data there exists some relationship connections.
According to croxton and Cowden" The appropriate statistical tool for discovering and measuring the relationship of quantitative nature and expressing it in brief formula is known as correlation.
The coefficient of correlation measures the degree of relationship between two set of figures. As the reliability of estimates depend upon the closeness of the relationship it is imperative that utmost care be taken while interpreting the value of coefficient of correlation otherwise fallacious conclusion can be draw.