The Controlled Input Method is a brainstorming technique that adopts both a nominal and an interactive approach, which has been indicated in the literature as making creative group discussion sessions effective. A brainstorming session using the Controlled Input Method was carried out as the initial stage of the design process of a graduate-level design project in an educational setting. The brainstorming session was found effective in terms of productivity. The documentation of the session was qualitatively and quantitatively analysed for identifying the factors contributing to the session’s effectiveness. The analyses revealed seven discussion topics on the problem area, three solution areas gathering design ideas, seven statement types used in the documentation, and two problem frames situating the discussions, contributing to the identification of fifteen strategies used in collective design reasoning. The strategies are discussed in terms of content creation, problem exploration and idea generation as functions of design reasoning, and in reference to divergence, convergence, quantity, situatedness, and goal-orientedness, as indicators of effectiveness.