Bivariate analysis is used to obtain a better understanding of the relationship between two variables x and y, such as the length and width of a fossil, the sodium and potassium content of volcanic glass, or the organic matter content along a sediment core. When the two variables are measured on the same object, x is usually identified as the independent variable, and y as the dependent variable. If both variables have been generated in an experiment, the variable manipulated by the experimenter is described as the independent variable. In some cases, neither variable is manipulated, and thus, neither is independent.