While studying the characteristics of a series involving only a single variable , which we generally call as univariate distribution , we have to restrict ourselves to find the measure of central tendency , its variation and skewness etc. Whenever the idea of bivariate distribution or a multivatiate distribution comes into our mind the very first desire which arises is to know as to how they are related to each other . If some how or desire which arises is to know as to how they are relate to reach other our next issue will be to know the extent of their relationship . The relationship between two series which are quantitatively measured is know as correlation . Here we must distinguish between the two words correlation and association . Correlation is the relationship between two or more series which are quantitatively measured where as association refers to the relationship between the series which are qualitatively measured .
Although we have to use some mathematical formulae to know and confirm extent of correlation between two or more variables but the issue is so common and popular that we do not have to stain ourselves to judge the behavior of the series with regards to their relation with each other .
Sir francis galton was one of the first of few statisticians who developed the technique of establishing the concept of correlation graphically in 1896 . Karl pearson was the one who introduced the formula based on mathematical treatment and established the extent of correlation . It was logically as well as other wise proved that if two or more quantities vary in sympathy so that movement in the one tend to be accompanied by corresponding movement in the other then they are said to be correlated