Daily UX Crash Course — User Psychology: 29 of 31

If you’re working in UX design, uncertainty is a part of your life. User psychology is not about making something work. We are trying to make it work better. So today we will learn about: 

The Probability of Interaction


(Just starting the User Psych Crash Course? Start here.)


When you measure user behavior, it helps to understand what a group looks like, statistically. Numbers can tell you a lot about your design, but you need the right perspective.


1% of people will do anything.

Many years ago, a colleague of mine named a rule after me:

Marsh’s Law: “Every feature will eventually be abused to its maximum abusability.”

I.e. — if something can be used, it will eventually be used. That doesn’t make it good.

On a page with 25 banners that nobody cares about, a few people will still click them.

On a site with 8 levels of navigation, most of those pages will be visited once or twice.

And I am still amazed that Facebook “Like” buttons get clicks on porn sites — or so I have heard.

It will be tempting to keep features because “some” users use it. Resist that temptation.

Every time someone clicks a shitty feature, they aren’t clicking something more useful.


90% is everybody.

UX is all about probability. Probability is the chance that a user will do something.

If you make a design so effective that the majority of people do what you hoped, that’s great!

If 90% of your users are aware that they can pay or register to get an upgrade, your design is great.

If 90% of users leave your site without clicking anything, it’s an emergency.

The only time I have seen 100% of users do something, it was a technical error or there was only 1 visitor.


Probabilities are not Intuitive.

If 10% of people click on your landing page, and then 80% of those people make a purchase, is the design good?

Actually, no.

Many people will look at the conversion rate and say “Wow! 80% of people make a purchase!”

Then the whole team will go have drinks or play in the inflatable castle or whatever people do at your company to celebrate.

However, as the UX designer, you should not go to that party. You’re losing 90% of potential sales.

But you aren’t losing them in the check out. You are losing them at the landing page.

If 40% of people click on the landing page and 40% of them make a purchase, you would actually sell twice as much, even though your conversion rate is half.

80% of 10% = 8%

40% of 40% = 16%

Almost every company I have worked for has had a problem like this, but very few people notice. It can literally be a million-dollar-mistake.


Tomorrow we will learn about ways that your information architecture can look like psychology, but it isn’t: Structure vs. Choice.