This paper is the continuation of a series of related work about experimentation of alternative ways of interaction with computers for disabled people (concretely with users suffering from cerebral palsy). In order to define an effective methodology to evaluate the usability and/or accessibility levels for this user profile, here, we start studying and evaluating (a) different interactive devices and interaction techniques that enable the use of computers for this people and, (b) existing metric techniques from human-computer interaction field. With this basis the article proposes a new evaluation method that simplifies the existing ones reducing the complexity to the minimum. This proposal is extensively explained and field research will soon be started