The document discusses different types of correlation between two variables: positive correlation, negative correlation, and no correlation. It defines correlation as a statistical measure of the linear relationship between two variables. Different methods for measuring correlation are described, including scatter diagrams, Karl Pearson's coefficient of correlation, rank correlation, and autocorrelation. Karl Pearson's coefficient yields a numerical value between -1 and 1 to indicate the strength and direction of linear correlation. Rank correlation is used for qualitative variables by assigning ranks and finding the correlation between the ranks.