Samriddhi Agarwal's Key Ideas from The Design of Everyday Things
by Don Norman

The "seven stages of action" represent the cycle of human action: forming a goal, creating a plan, specifying the steps needed for the plan, and performing those steps (execution); then perceiving the results, interpreting their meaning, and comparing the outcome against the original goal (evaluation). Design should consider all seven stages, making it easy for users to form goals, execute actions, and understand the results. A good design bridges the "gulfs" between execution and evaluation, ensuring the user can easily operate the product and get a clear result.

"Goal-driven" behavior starts with a conscious intention or objective that triggers a series of actions to achieve it. In contrast, "event-driven" behavior begins with an environmental stimulus, or external event that is noticed and causes a person to act to fulfill or re-establish a goal. For example, the lack of light would be a stimulus that triggers the event-driven behavior of finding a way to get more light. Understanding these types of actions can help guide design choices.

There are three levels of processing that shape our interactions with products and the world: visceral, behavioral, and reflective. The visceral level involves our immediate, subconscious reactions to sensory input (e.g., like/dislike). The behavioral level involves our learned, subconscious actions (e.g. skills, habits). Finally, the reflective level deals with conscious thought, deep understanding, and higher level emotions. Designers must consider all three to create products that are not only functional but also appealing and enjoyable to use.

"Knowledge in the head" refers to what we actively remember - the information we carry in our minds. "Knowledge in the world" refers to information we get from the environment, such as labels, signs, and objects that guide us. Designers must consider the balance between the two. Products should not rely entirely on user memory but should also offer visual and physical cues to aid usage and prevent error.

"Slips" are errors in execution, where the user intends to do one thing but accidentally does another (e.g., putting the coffee cup in the refrigerator instead of pouring the milk in the cup). "Mistakes" are errors in planning where the wrong goal or plan is selected (e.g., using the wrong rule or misdiagnosing a situation). To minimize these errors, designers must focus on clear communication, easy-to-understand interfaces, and designs that prevent accidental actions. For slips, forcing functions and constraints are useful while reducing mistakes requires clear system understanding.

The "double diamond" design model breaks down the design process into four phases: discover, define, develop, and deliver. The first two phases, discover and define, emphasize understanding the problem and its context, involving research and user analysis to generate a clear understanding of the problem. The second two phases, develop and deliver, emphasize generating ideas and then creating the actual product. This model encourages designers to explore multiple possibilities at each step before converging on the best solution.