How to Evaluate Interaction Cost and Boost User Experience

How to Evaluate Interaction Cost and Improve User Experience?

Every product designer must master three core skills: product thinking, visual design, and interaction design. While visual design and product thinking can be self‑taught, interaction design—especially understanding and reducing interaction cost—is the most challenging.

Defining Interaction Cost

Interaction cost is the total physical and mental effort a user must expend to achieve a goal. Nielsen Norman defines it as the sum of the user’s bodily and cognitive effort.

Designers aim to keep interaction cost as low as possible, but more user scenarios increase information‑architecture complexity, leading to higher cost.

Physical vs. Psychological Interaction Cost

Interaction cost can be split into Physical Interaction Cost (PIC) and Mental Interaction Cost (MIC). Many junior designers mistakenly equate cost with click count, ignoring the broader picture.

Identify Key User Flows

Best practice is to locate the primary user flow (the "red route") and, if necessary, sacrifice secondary flows to reduce cost for the main scenario. Red Route Analysis (RRA) helps evaluate which use cases are most critical.

Complexity Conservation Law

According to Tesler’s law, every system has inherent complexity that cannot be eliminated. Good design shifts this complexity from the user to the system, even if it means developers spend extra time simplifying the product.

Psychological Interaction Cost (MIC)

MIC consists mainly of attention and memory demands. Excessive attention‑draining elements (pop‑ups, unrelated animations) increase MIC and reduce usability.

Evaluating Attention

Eye‑tracking studies (ETS) can reveal where users look and how long they fixate, helping identify distracting elements.

Working Memory

Miller’s Law states that short‑term (working) memory can hold 5‑11 items. Tasks should avoid overloading users; chunking information reduces mental load.

Physical Interaction Cost (PIC)

PIC factors include distance to target, target size (Fitts’s Law), number of inputs, and overall task steps. Task analysis and metrics like task time help evaluate PIC.

Interaction Paths and Motivation

Users may choose different paths based on expected utility = expected benefit – expected interaction cost. Even if a path has lower physical cost, higher MIC can make users prefer a more familiar route.

Conclusion

Understanding and evaluating interaction cost is essential for modern product designers. Reduce cost wherever possible, but when trade‑offs are required, prioritize the primary user scenarios and shift remaining complexity to secondary flows.