Correlation is the extent to which two or more variables change together. Pairs of observations can be plotted as a series of points on a graph. The correlation coefficient measures the extent to which the points on the resulting scatter diagram form a straight line. The correlation coefficient varies within the range +1 (where an increase in one variable is associated with a corresponding increase in the other) to -1 (where an increase of one variable is associated with a corresponding decrease of the other); a coefficient of 0 indicates no linear relationship between the two variables. Multivariate analysis can be used to investigate the correlation in several variables.