In determining the relationships among prices at the farm, wholesale and retail market levels, it is necessary to determine the lengths of time between infuences and adjustments. For instance, it is commonly assumed, as Bowley pointed out many years ago, that changes in wholesale price will lead to changes in retail price at some point in the future. Within this time period, more than one change in a particular price may have affected the price at another level. The number of past observations of one price variable that describe another is called the lag length. Lag lengths may be determined based on theory, previous studies, biological restrictions or model selection tests.