An interval number is a real, closed interval .
Arithmetic rules exist for interval number.
In all the following sections an interval for the variable x will be denoted by . All the procedures described in the following sections use the free interval analysis package in which the basic operations of interval analysis are implemented. This package uses a fixed precision arithmetics with an accuracy of 10 -16 . Different types of data structure are implemented in this package. All basic arithmetic operations can be used on interval-valued data using the same notation than for fixed numbers.