The measures of Central Tendency and Dispersion are used to compare two or more series. But it is not the comparison that is always wanted. Sometimes we need to find relation between two or more variables; particularly in Socio-Economic type of problems. Whenever we think of Bivariate or a Multivariate distribution, the idea of their relation comes into our mind at once. These variables may be ‘Price and Supply’, ‘Income and Saving’, ‘Income and Investment’ or ‘Income and Consumption’ etc. The relationship between two series when measured quantitatively is known as Correlation. Due to this correlation, change in the value of one series brings about a change in the value of other series correspondingly. This change may be in the same or in the opposite directions; and magnitude of change may not be the same. We may conclude that ‘Correlation is a Statistical Technique which shows the relationship between two or more variables’.
According to Croxton and Cowden, “If the relationship is of quantitative nature, the appropriate statistical tool to discover and measure that relation and express it in the form of a brief formula, is correlation.”
according to Simpson and Kafka, “Correlation analysis deals with the association between two or more variables.”
According to A.M. Tuttle, “Correlation is an analysis of the co-variation between two variables.”
According to L.R. Connor, “If two or more quantities vary in sympathy so that movements in the one tend to be accompanied by corresponding movements in the other, then they are said to be correlated.”
Although Karl Parson was the first to establish the mathematical formula but Sir Francis Galton was the first to develop the technique the obtain it graphically.