Use Correlation to measure the strength and direction of the association between two variables. Minitab offers two methods of correlation: the Pearson product moment correlation and the Spearman rank order correlation. The Pearson correlation (also known as r), which is the most common method, measures the linear relationship between two continuous variables.
For example, an engineer at a manufacturer of particle board wants to determine whether the density of particle board is associated with the stiffness of the board. The engineer performs a correlation analysis using the Pearson correlation to evaluate the strength and direction of the linear relationship between density and stiffness.
If you are not certain whether your variables are linearly related, you should create a Scatterplot. If the relationship between the variables is not linear, you may be able to use the Spearman rank order correlation (also known as Spearman's rho). The Spearman correlation measures the monotonic relationship between two continuous or ordinal variables. For more information, go to A comparison of the Pearson and Spearman correlation methods.