Interval Regression in Gretl

Hey everyone,

i'm studying economics and i'm part of a project that did a survey on willingness-to-pay data concerning environmental goods in germany.

In order to get the willingness-to-pay of a questioned person we used a payment card that contains the following values: 0, 1, 5, 10, 20, 40, 70, 100, 150, 200, 300, 400, 500, >500.

We would like to use an interval regression in order to get to know for example the influence of income on the willingness-to-pay. So we've got a dependent limited variable.

We tried to use gretl, but it is very difficult to get specific informations concerning this software. Because of that i ask here if anybody has an idea how to solve this problem in Gretl.

We already created the variables boundlow and boundhigh (and logboundlow, logboundhigh) in order to censor the data. If we do an interval regression in gretl, it actually works, BUT: only without using the value >500. I have no idea how to implement the last interval which has an open end [500, infinity].

If anybody needs further information just tell me. And sorry for my english...