The best designers have an accurate mental model of their users. In their mind, they can put their product in the hand of the 30 year old floor manager at the Borders at the mall and predict what will happen with shocking accuracy.

They can predict it because they've watched the floor manager interact with past products in the usability lab dozens of times.

And because they have watched that person dozens of times, they know that even thought they can predict extremely well how that person will react, they know they have to run tests anyway, because the best designers know how often they've been surprised in the past.

The best designers have a great mental model of their users, and know when to use it.

The worst designers just use themselves as a model of their users, and their designs are accordingly screwy.

Sometimes it also helps having an accurate mental model of yourself. Not as common as one could assume.

Well with regards to testing,

Apple don't do any kind of usability testing in any normal sense.

They build products they want themselves and then they see what happens. The feedback they get back from that is the feedback they use.

Of course Apple does usability testing and has since the beginning. Here's a full guide on how they do it:

http://developer.apple.com/mac/library/documentation/UserExp...

Obviously they use their own judgment and market feedback as well, but the notion that they launch products without testing them first is crazy.

No one is saying they aren't testing them. Of course they are.

They are just not testing them in any normal sense.

I.e. they don't invite users to test it in usability labs or any other typical test environments.

http://www.businessweek.com/magazine/content/05_12/b3925608....

That was the paragraph that jumped out at me too, because he got that exactly wrong. The iPhone designers clearly did understand their users, all 120 million of them. My wife is a total technophobe and the iPhone has finally connected her to her friends online.

I still deal with this issue working with techies who have iPhones and develop on iPhones. There are 100,000 iOS developers. 120 million iOS devices. So Mr Platt may be correct about developers in general, but he certainly got it wrong about Apple.

I must disagree that the best designers are like their users. The best designer is someone who can design a user interface for 120 million. Yet most iPhone users can't. It takes a different mindset to design a UI than to use it.