In the previous chapter we developed a model that used a number of functions. While the built in functions can be used to represent many relationship they are not always enough. It is often easiest to create special functions with the properties or shape that you want.
Lookups allow you to define customized relationships between a variable and its causes. An equation can be defined with a specially-constructed function:
y = my function(x)
The output variable y is changed by input variable x through the Lookup function my function, which has a (typically nonlinear) shape that you specify.
Lookups are also known as Lookup Functions, Graphical Functions, Lookup Tables, or simply Tables. They can be constructed as a table of numbers (in the Equation Editor), or as a graph (in the Graph Lookup Editor).