We have discussed various methods for studying the association between two variables, particularly the linear relationship. The scatter diagram gives a visual presentation of the relationship and is not confined to linear relations. Measures of correlation such as Karl Pearson's coefficient of correlation and Spearman's rank correlation are strictly the measures of linear relationship. When the variables cannot be measured precisely, rankcorrelation can meaningfully be used. Correlation gives us an idea of the direction and intensity of change in a variable for changes in the related variable.