When for every value of a variable X we known a corresponding value of a second variable Y, i.e., the data is in the form of paired measurements, then we are interested in the relationship of these two variables. The analysis of univariate data in case of economic, social and scientific areas becomes insufficient. So, in some situations, say production price, height of father and son, marks obtained in two subjects, height of husbands and their wives, a series of expenses on advertisement and sales. If change in one variable is accompanied (or appears to be accompanied) a change in other variable and vice-versa, then the two variables are said to be correlated and this relationship is known as correlation or covariation.
Read this topic