Module 4: From intuition to evidence

Justifying without manipulating

Learning objectives: By the end of this module, you will be able to:

Predict how easy it is to reach a target on screen based on its size and distance (Fitts's law).
Translate a personal preference into a grounded hypothesis others can argue with.
Recognize when citing a law becomes a way of closing the conversation instead of opening it.

Estimated time: 1-1.5 hours

So far you have a handful of phenomena and their limits. What's missing is the thing that makes them useful in your actual work: using them to justify decisions in front of other people. Because in practice you're not going to apply psychology alone in front of your screen; you're going to wield it in a meeting where somebody says "I like it better this way".

This module has two parts. One concrete, almost measurable law, Fitts's on movement toward a target, which serves as an example of how a law used well predicts rather than just opines. And a skill that isn't cognitive but communicational: turning "I like it" into an argument that can be discussed and, above all, tested.

That second part is communication psychology, and it's aimed at your team, not at the user. I'm flagging it because it's easy to mix them up.

4.1. The wastebasket analogy

Fitts's law is one of the few "laws" in this course you can almost calculate. In plain terms it says: the time it takes to reach a target depends on its size and how far away it is. The bigger and the closer, the faster and with fewer errors.

Think of it as throwing a piece of paper into the wastebasket. A big bin right next to you: you sink it without looking. A small bin across the office: you miss half the time. Nothing but size and distance.

In an interface, your buttons are wastebaskets. The button you most want people to use should be the biggest and the easiest to reach. The screen's edges and corners work like bins of infinite size: the cursor stops on its own there. It sounds obvious, but it's one of the few things in UX you can defend with a number before testing.

The limit of Fitts's law: it was formulated for pointing tasks with a device; with touch gestures and small screens the math gets complicated. And watch out for reading it as "every important button should be big and easy to reach": a huge, accessible "Delete account" is a Fitts triumph and an experience disaster. Easy to reach is not the same as worth reaching.

4.2. From "I like it" to a testable hypothesis

Step 1: name the decision in dispute

"The payment button goes small and up top" versus "big and at the bottom, within thumb reach".

Step 2: find the law that applies

Here, Fitts: on mobile, a target that's large and close to the thumb is reached faster and with fewer errors.

Step 3: turn the law into a prediction

"If we make the button bigger and move it down, we expect fewer tap errors and a faster checkout." Now it's a claim that can be confirmed or refuted.

Step 4: test, don't stop at the citation

Its limit, and this is the heart of the module: citing the law doesn't win the argument, it organizes it. The law generates the hypothesis; the test resolves it. Using "Fitts's law says that…" to shut down the debate is an abuse of authority, and on top of that it leaves you learning nothing. Evidence used well opens a test, it doesn't replace one.

4.3. Case study: "AgendaPro" (B2B tool)

At AgendaPro, a scheduling tool for teams, the design meeting got stuck on the confirm-appointment button. The product lead wanted it discreet ("it looks more elegant small"); the designer wanted it prominent. Half an hour of "I like it".

The researcher reframed it. Instead of arguing about taste, she proposed the hypothesis: "The main action on this screen is confirming. By Fitts's law, a bigger, better-placed target is reached faster and with fewer errors. The hypothesis is that making it bigger lowers confirmation errors. Let's test it for a week."

The law didn't close the discussion: it turned it into something measurable. The test showed a drop in errors, and the decision stopped depending on who had the most seniority in the room.

The detail that matters: if the test had come out flat, the law would still have done its job. It wasn't there to be right, it was there to turn a fight about opinions into a question with an answer. That is, as far as I've seen, the best use you can get out of psychology in a meeting.

4.4. Application activity (15 minutes)

Take a recent design decision you made "on intuition".

Try to name a law or principle behind it.
Turn it into a prediction that can be tested.

Suggested solution: if you can't find any law behind it, that doesn't mean the decision was wrong. It means it was aesthetic or contextual, and it's better to say so than to dress it up as science. Being honest about what's evidence and what's taste is part of the craft, and it saves you arguments where both sides pretend to have data.

4.5. References

Fitts, P. M. (1954). The information capacity of the human motor system in controlling the amplitude of movement. Journal of Experimental Psychology, 47(6), 381–391.
MacKenzie, I. S. (1992). Fitts' law as a research and design tool in human-computer interaction. Human-Computer Interaction, 7(1), 91–139.

Additional material. From MIT's Introduction to Psychology (9.00SC): Science & Research, on how evidence is evaluated in psychology. If you're going to watch a single session of the whole course, make it this one: it teaches the attitude this module asks of you.