A mathematical expression of the inverse ratio between the quantity of a substance and the distance from its source; the rate at which interaction declines as the distance from the source increases. This function is a specific example of linear regression analysis and can be used to describe the relationship between the amount of a given commodity found at any point and the place from which it was exported. The patterns and mathematical expressions help to distinguish different forms of trade and exchange. In general, distance-decay varies with the value of the object traded, with the richer items spreading further from the source.