A parameter is a property of the legislation that changes over time.
Unlike a variable, a parameter is not specific to a specific entity (person, household…).
For instance:
the amount of the minimum wage;
the amount of family allowance per children;
the marginal tax scale used to calculate the income tax;
the maximum age at which a person is entitled to some benefits;
the list of regions in which a tax is applicable…
Parameters are used in formulas to calculate variable values.
It is always advisable when modelling legislation in OpenFisca to create a parameter for any number described in the law and used in formulas rather than “hard coding” the number within a formula. Thus, if the parameter changes over time, OpenFisca will automatically retrieve the right parameter value for your calculation period.