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There are situations where it is desirable and necessary to study two or more variables at a time. For example, if we study the yearly sale of the product of a company, we would also like to know its profit, because from that we can find the nature and degree of relation between these two variables 'sale of product' and 'profit'.

The yearly rainfall and yield of rice in some region, price and demand for an item, income and expenditure of a family, height of a father and his son, age of a husband and wife, are some well known examples where we see the relation between paired variables.