Some ethical evaluation models for Intelligent Assistive Technologies (IATs) have been presented in the literature. However, it has been proven that the vast majority of IATs were designed in the absence of explicit ethical values and considerations. In order to shed light on this problem, this paper presents a review of the literature on ethical values and considerations regarding IATs. According to this review, four challenges are identified to facilitate the evaluation of ethical models in IAT design: (i) Rational method, (ii) uncertainty evaluation process, (iii) evaluation results and aggregation process, (iv) software tool. These challenges are analyzed in order to propose an initial ethical model based on linguistic decision analysis.