The use of the product is randomly taken into account in the final phase of the design process, which leads to certain iterations and difficulty in use.Generally, the designer has taken into account customer requirements without really knowing the end user, sometimes our end users.By analyzing the different users, it is a question of measuring the functionality of the product for each of them. The various functional analyses must take into account the needs of all these users. Thus, it is necessary to identify: the expectations and cognitive barriers of each user in a competitive environment and to consider the evolution of the use and performance of the product and its interaction with its users.In this article, it’s about listing the product features for each user case. The different functional analyzes must present the expectations for each use case. Thus, in an order of the product use life cycle, it is necessary to identify the purposes and behaviors of users and the level of constraint of each feature. By comparing the functions between them, it is a question of identifying the contradictions or not that can have an effect on the design of the product. A concrete example (baby car seat), is proposed in this article in order to target the different users from the functions and to identify the contradictions that can be resolved by the TRIZ principle.