  Extensions of previous linear regression models for interval data are
presented. A more flexible simple linear model is formalized. The new model may
express cross-relationships between mid-points and spreads of the interval data
in a unique equation based on the interval arithmetic. Moreover, extensions to
the multiple case are addressed. The associated least-squares estimation
problem are solved. Empirical results and a real-life application are presented
in order to show the applicability and the differences among the proposed
models.
