A nonlinear correlation is a relation between elements where the variables associated with each do not change in a constant ratio to each other; the proportionality to the change between elements can change over time, thus mapping out a graph that is not a straight line. For example, there is a nonlinear correlation between the size of a factory and the cost of maintaining it, to operate a plant of one thousand square meters would not cost twice as much as that of five hundred square meters, because of the synergies to economies of scale.