Software Engineering Laws - Design & UX

Jakob’s Law

Users spend most of their time on other sites, so users prefer your site to work the same way as all the other sites they already know

This law is a powerful mandate for convention over unnecessary novelty in user interface design. It posits that users have built a powerful mental model for how the web works based on their cumulative experience across thousands of other applications. Your application is a guest in their digital life. To be a good guest, you must follow the house rules they have already learned. Violating established patterns forces users to learn a new model, creating friction and cognitive load that drives them away.

Why it happens:

• Conservation of Cognitive Energy: Users do not want to spend their limited mental energy learning how your unique navigation menu works. They want to spend it achieving their goal (e.g., buying a product, reading an article). By using familiar patterns, you lower the cognitive cost of using your application, making it feel intuitive and effortless.
• The Power of Habit: Common interactions are deeply ingrained habits. Users instinctively look for a logo in the top-left to go home, a shopping cart in the top-right, and a search bar near the top of the page. Deviating from these conventions doesn’t feel creative; it feels broken.
• Transfer of Expectation: When a UI element looks like a standard component (e.g., a dropdown menu), users expect it to behave like one. If your “creative” dropdown violates those behavioral expectations, you create confusion and erode the user’s trust in your interface.

What to do about it:

1. Default to Convention: When designing any standard UI component—be it a login form, a data table, or a settings page—your first step should be to research how the most popular applications handle it. The problem has likely already been solved. Adopt the prevailing pattern.
2. Focus Innovation on Your Core Value: Do not waste your team’s limited creative budget on reinventing the date picker. Save your innovative energy for the parts of your application that are truly unique to your domain and deliver your core value proposition. For everything else, be boringly predictable.
3. Use a Design System to Enforce Consistency: A robust design system is the most effective tool for implementing Jakob’s Law at scale. It provides a canonical, pre-vetted, and conventional component for every common UI need. This prevents individual teams from introducing novel (and likely confusing) patterns, ensuring a consistent and intuitive experience across your entire product.
4. Use Usability Testing to Confirm, Not Discover: If users are consistently struggling with a basic element in your UI, the problem is not the user. It is almost certainly a violation of a design convention they have learned elsewhere. Use testing to identify and eliminate these moments of friction, bringing your design back in line with user expectations.

Fitts’ Law

The time to acquire a target is a function of the distance to and the size of the target

This is a predictive model of human movement that has profound implications for all interface design. It states that hitting a target—be it a button on a screen or a physical object—is faster and easier when the target is larger and closer. This isn’t a suggestion; it’s a fundamental principle of human biomechanics. Ignoring it results in interfaces that feel clumsy, slow, and frustrating.

Why it happens:

• Motor Precision: Small, distant targets require more precise motor control and finer adjustments, which takes more time and increases the probability of error. Large, close targets can be acquired with faster, less precise movements.
• Cognitive Effort: While rooted in motor skills, the law has a cognitive component. Interfaces that adhere to Fitts’s Law require less conscious thought to navigate. Users don’t have to carefully aim their pointer; they can move quickly and confidently, reducing mental friction.
• The Magic of Edges: The edges and corners of a screen are effectively “infinitely large” targets in one dimension because the pointer cannot overshoot them. This makes them the fastest possible targets to acquire, a principle leveraged by macOS’s top menu bar and the Windows Start button.

What to do about it:

1. Prioritize by Size and Position: Make your most important and frequently used UI elements (e.g., the primary call-to-action on a page) the largest and most prominent targets. Place them in positions that are easy to reach from the user’s likely previous point of interaction.
2. Make Destructive Actions Harder to Hit: Conversely, use Fitts’s Law for safety. Make dangerous or irreversible actions (like “Delete Account”) smaller, place them further away from common interaction areas, or require an extra click. This deliberate increase in target acquisition time acts as a natural speed bump, preventing catastrophic errors.
3. Minimize Travel Distance: Group related elements together. A “Save” button should be located near the form fields it acts upon, not at the opposite corner of the screen. In a mobile app, place primary navigation elements within easy reach of the user’s thumb.
4. Apply it Metaphorically to API Design: Fitts’s Law can be extended beyond GUIs. Think of the data a client needs as the “target.” A “chatty” API that requires multiple round trips to gather all necessary information has a very large “travel distance.” A well-designed API (e.g., using GraphQL or a Backend-for-Frontend pattern) allows the client to acquire all the data in a single call—a large target with zero distance—making the entire interaction vastly more efficient.