Design is becoming the decisive factor in whether a product is a commercial success, like Windows XP, or a critical failure, like Microsoft Bob. To leverage this factor we need to have a greater understanding of the cognitive processes behind Interaction Design. While there are a wide array of disciplines that fall under the umbrella of design, there are several cognitive processes that are common to all strata of design. Decision Making has been identified as an important factor in the design process but remains woefully under-explored. This paper aims to understand Design Decision-making (DDM) in the light of more recent developments in the wider decisionmaking field. Two studies were conducted, consisting of an initial theoretical thematic analysis to update the outdated models of design decision-making, and a follow-up quantitative study to validate the findings of the first study. Results indicate that while the current models of DDM do well to explain elements of the decision-making process they do not account for such things as the persistence of analogies across all stages of the decision-making process.

Motivation-The Recognition Primed Decision-Making model (RPD) (Klein, 1999) has been used to describe Design Decision-Making (DDM). Originally the RPD model was a multi-stage model but it has matured into three variations to better describe differing situations that one could encounter. These variations have not been reflected in the DDM literature. Research approach-Theoretical Thematic Analysis was conducted, based on six semi-structured interviews with Interaction Designers. Findings-Three themes were identified: 1) the use of analogies, 2) the impact of constraints, and 3) the use of mental simulation. Value-This study aims to update the current understanding of DDM to be more in line with the current NDM literature. Take away message-The third variation of the RPD model is best at encapsulating the DDM process exhibited by participants, although none of the variations could account for results such as the persistence of the initial analogy.