Attention and memory are the two most important components of mental interaction costs.  If tasks require excessive attention or memory, they will have proportionally higher mental interaction costs.  Some elements that can divert attention and increase these costs are pop-up banners, attention-grabbing visuals, and motion that is not related the the task.  Users can be distracted easily, so it is important not to pull their attention elsewhere.

Attention and Memory

In order to evaluate what users are paying attention to in your interface, you can do an eye-tracking study (ETS).  If the ETS reveals too many task-irrelevant saccadic movements, you likely have a distracting interface.  From the results, you can understand if users are missing critical elements in your interface, discover what's distracting the user, and find out what's unnecessary.

Evaluating Attention

Mental interaction costs can include complex navigation and dense instructions, as well as unconventional interaction patterns. They can sometimes be overlooked by people who are only paying attention to physical interaction costs.

- Attention and Memory

- Evaluating Attention

- Working Memory

New York University

As a product covers more use cases, or the sequence of steps starting with the user's goal and the end result when the goal is achieved, it becomes more difficult to lower interaction costs.  Having more functionality and use cases adds complexity to navigation and information architecture.

- Find the Critical User Flows First

- How Tesler's Law Helps Reduce Interaction Costs

- Mental Interaction Costs

- Physical Interaction Costs

- Common Pitfalls to Avoid

How to Evaluate Interaction Costs and Improve User Experience

https://www.freecodecamp.org/news/interaction-design-evaluate-interaction-costs-improve-ux/

Interaction Design – How to Evaluate Interaction Costs and Improve User Experience Reference

One way to reduce the overall interaction cost of a product is to identify the primary use cases and reduce the interaction costs there at the expense of the secondary and tertiary use cases.  To determine which use case interaction flows are most important to your users, you can conduct user interviews and create a Red Route Analysis (RRA).  Red routes are critical and often used, provide the most value and tie to product metrics.

Find the Critical User Flows First

Tesler's Law of Conservation of Complexity states that all systems have an inherent complexity that cannot be removed or hidden.  Good designs make it so that the complexity burden is on the shoulders of the product rather than the user.  The complexity burden should be shifted towards the least important use cases.  However, interfaces should not be simplified to the point of abstraction.  When the system has taken care of as much of the inherent complexity as possible, the rest of it should be shifted away from the primary use cases to the secondary and tertiary use cases.

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