In this article we compare five alternative projects for the requalification of an abandoned quarry. The starting point for this paper was a request made by a decision maker. It was not for help in making a decision as such, but rather for a comparison of different projects. In particular, we are interested in ranking the considered projects on the basis of six different criteria. An extension of the Electre III method with interactions between pairs of criteria was applied in the research. A focus group of experts (in economic evaluation, environmental engineering, and landscape ecology) was formed to be in charge of the process leading to the assignment of numerical values to the weights and interaction coefficients. We report on the way the process evolved and on the difficulties we encountered in obtaining consensual sets of values. Taking into account these difficulties, we considered other sets of weights and interaction coefficients. Our aim was also to study the impact on the final ranking of the fact that these numerical values, assigned to the parameters, were not perfectly defined. This allowed us to formulate robust conclusions which were presented to the members of the focus group. 
Dealing with a multiple criteria environmental problem with interaction effects between criteria through an extension of the Electre III method