Mathematicians sometimes distinguish between propositional constants, propositional variables, and schemata. There are a number of inequalities involving the expected values of functions of random variables. Probit models offer an alternative to logistic regression for modeling categorical dependent variables. Monomials in "n" variables define homogeneous functions . Closures are functions evaluated in an environment containing bound variables. Derivatives may be generalized to functions of several real variables. Let formula 8 be a function with formula 9 variables. Covariance is a measure of linear relationship between the random variables. Let be a function of two real variables. It describes the local curvature of a function of many variables. Execution proceeds by attempting to find values for the variables which satisfy all declared constraints. Inferential statistics includes techniques to measure relationships between particular variables. Each cardinal function is a monotonic function of each of its "natural" or "canonical" variables. This helps simplify the analysis of dependencies among variables. A common reason is the omission of relevant explanatory variables, or dependent observations.