A variable rule of /s/ deletion that operates in many varieties of Spanish is examined. A descriptive apparatus is posited that may be helpful in systematizing quantitative data from studies of phonological variability. Empirical data from the speech of informants show that ordering relationships among constraints on variable phonological rules may be quite complex. Factor groups may be ordered on a strength scale by means of a cross-products method. First and second order constraints are illustrated. Factors within a factor group may also be ordered. It is argued that this ordering should be determined on the basis of the relationship of the individual to the group. Three types of constraints are suggested: primary (all individuals conform), secondary (a significant majority of individuals conform), and "statistical" (the group as a whole conforms but individuals vary randomly). It is suggested that this classification also describes certain aspects of the diachronic development of the adoption and spread of a variable rule. (Author/JB)