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.

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

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.**

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.