# Variables and formulas

A variable is property of a person, or an entity (e.g. a family).

For instance:

• The birth date of a person
• The amount of basic income (in France, RSA) a family can get in a month.
• The amount of income tax a household has to pay in a year.
• Whether a family is living in Paris, or not.

### Input variables

Some variables can only be given as inputs of a simulation. For instance, the birth date of a person.

### Formulas

Other variables can be calculated thanks to a formula.

A formula is a function that calculates the value of given variable, for a given period. To do so, it performs (usually arithmetic) operations on the values of other variables, the formula dependencies.

For instance:

• The basic income of a family can be calculated from its income, and some other information about its situation.
• The income tax of a tax household can be calculated the same way.

It is important to note that all variables can be used as inputs. This means that even if the basic income can be calculated from other variables, I can, for a given simulation, provide it as an input. Then, if another formula asks for the value of basic income for a month, the input value will be returned, and the basic income formula won't be executed.

### Default values

When OpenFisca is not able to calculate the value of a variable for a requested period, it returns a default value.

The default value of a variable is returned:

• When the value of an input variables is requested, if this variable has not been set in the input for the requested period.

For example: Let's assume the input variable `student` default value is `False`. If the value of `student` for `2017-09` has not been set in the input of the simulation, then computing `student` for `2017-09` will return `False`.

• When the value of a variable with formulas is requested, if no formula is defined for the requested period.

For example: Let's assume the variable `basic_income`'s formula is defined starting `2015-01-01`, and its default value is `0`. Computing `basic_income` for `2014-01-01` will return `0`, while computing `basic_income` for `2015-01-01` will use the formula.

Legislation writers can define a specific default value for each variable.