Although a good’s own price is important in determining consumers’ willingness to purchase it, other variables also have influence on that decision, such as consumers’ incomes, their tastes and preferences, the prices of other goods that serve as substitutes or complements, and so on. Economists attempt to capture all of these influences in a relationship called the demand function. (In general, a function is a relationship that assigns a unique value to a dependent variable for any given set of values of a group of independent variables.) We represent such a demand function in Equation 1:

Equation (1) 

Qdx=f(Px,I,Py,...)

where Qdx represents the quantity demanded of some good X (such as per household demand for gasoline in gallons per week), P_x is the price per unit of good X (such as $ per gallon), I is consumers’ income (as in $1,000s per household annually), and P_y is the price of another good, Y. (There can be many other goods, not just one, and they can be complements or substitutes.) Equation 1 may be read, “Quantity demanded of good X depends on (is a function of) the price of good X, consumers’ income, the price of good Y, and so on.”