Nothing is perfect. So even for the most successful platforms, design needs to continuously evolve. But beyond mere gut instinct or that ephemeral talent of taste, how can you know when a design decision is good or bad? How can you be sure that change is actually for the better and not for the mere sake of change?

Editor’s Note 12/30/13 Happy (almost) New Year! We’re saying good-bye to 2013 by revisiting some of our favorite stories of the year. Enjoy.

As it turns out, Google’s Android User Experience team has a fantastic trick to settle these debates. In their presentation at Google I/O, the designers shared it with the rest of the world.

It takes 3 positive emotions to outweigh each negative one.

It starts with a mindset, that every design decision they make affects user emotion in a positive or negative way. Obviously, that means iteration needs to tip the scales to positive emotions, and to do that, the team follows the guiding rationale of psychologist Barbara L. Fredrickson, who discovered that it takes three positive emotions to outweigh every negative one.

With that 3:1 ratio in mind, Google sets up two jars (maybe metaphorically–we couldn’t quite tell). So when they go to tackle a problem, like, “How do you signal to a user that they’ve swiped to their final page of apps?” they examine the costs and benefits of this solution in a 3:1 ratio.

How do they know what’s good and bad? The Android team has actually published their guiding rubric of 17 design principles to “Enchant, Simplify and Amaze” the user. You can read about them all here, but the CliffsNotes version is that Google creates design mantras from the point of view of the user, like “keep it brief,” “delight me in surprising ways,” and “it’s not my fault.” Each time an Android feature lives up to these expectations, they get a single marble in the good emotion jar. But every time they fail, that bad feature produces three marbles in the bad emotion jar. The marbles illustrate that bad ideas stack up quickly.

The marbles illustrate that bad ideas stack up quickly.

So let’s go back to that “How do you signal to a user that they’ve swiped to their final page of apps?” problem. Any solutions? Well, what if Google did nothing? Literally, just have the UI do nothing to communicate that final screen. That would break two of their design rules, creating two negative emotions. (“I should always know where I am”–because am I on the last screen or what?–and “it’s not my fault”–because why isn’t this working??)

Alternatively, they could have a pop-up notification explaining “this is the last screen.” But that still clashes with “it’s not my fault,” because, as the team explains, any pop-up notification is a bit like being nagged for doing something wrong. They also get three marbles for “only interrupt me if it’s important,” since reaching the last page of your apps isn’t going to destroy the world.