Engineers engage in design, and design requires decision making. Whether picking a color for a spoon designed to aid a person with physical challenges or choosing the material for the blade of a turbine; engineering design decisions are consequential for the design and how it performs upon implementation. To use a spoon, the person may need to like the color; and the material of the blade must be strong enough for an endurance task. Because design decisions are consequential, undergraduate engineering programs have a responsibility to prepare students as decision makers. Capstone design courses allow undergraduate engineering students to experience open-ended design projects before starting their professional careers. As such, capstone serves as an opportunity to develop students' ability to make decisions in an ill-structured setting. Typically, explicit instruction related to decision making includes an introduction to rationalistic tools, such as decision matrices or House of Quality. However, in the process of providing rationalistic tools to students, engineering education may be implicitly perpetuating the belief that engineers make decisions through rationalistic reasoning alone. In reality, other types of informal reasoning, such as empathic and intuitive reasoning, are utilized for decision making in ill-structured contexts such as engineering design. The beliefs that undergraduate students hold about decision making in the context of design is not well understood, and this work contributes to this gap in the literature. To learn more about students' beliefs about decision making, we collected qualitative pilot data in the form of both one-on-one, semi-structured interviews and written reflections from ten engineering students in capstone design courses at a large, Midwestern university. Participants provided accounts of previous decision-making experience within an engineering context. Throughout the data collection, they also were asked to describe how they perceive their own decision making with respect to an initial theoretical framework for types of informal reasoning, which included rationalistic, intuitive and empathic reasoning. Data were analyzed through open and values coding. This paper presents the resulting shift from our initial, three-part theoretical framework for informal reasoning (rationalistic, intuitive, empathic), to a revised, two-part framework for informal reasoning (logical and intuitive). This decision was based upon both our data analysis process and revisiting extant decision-making literature. The contribution that we provide in our revised framework for studying informal reasoning can be used by engineering educators and researchers to think critically about and investigate their own students' development as decision makers and their approaches to teaching decision making in the context of engineering capstone design. Future work will include data collection utilizing the revised framework and more in-depth data analysis with respect to students' beliefs about decision making, as well as the influence of a year-long capstone design course on their beliefs. [ABSTRACT FROM AUTHOR]