What metrics for GUI usability do you know?

Question

Of course the best metric would be a happiness of your users.
But what metrics do you know for GUI usability measurements?
For example, one of the common metrics is a average click count to perform action. What other metrics do you know?

Answer 1

Jakob Nielsen has several articles regarding usability metrics, including one that is entitled, well, Usability Metrics:

The most basic measures are based on the definition of usability as a quality metric:

• success rate (whether users can perform the task at all),
• the time a task requires,
• the error rate, and
• users' subjective satisfaction.

Answer 2

I just look at where I want users to go and where (physically) they are going on screen, I do this with data from Google Analytics.

Answer 3

Not strictly usability, but we sometimes measure the ratio of the GUI and the backend code. This is for the managers, to remind them, that while functionality is importaint, the GUI should get a proportional budget for user testing and study too.

Answer 4

check:

http://www.iqcontent.com/blog/2007/05/a-really-simple-metric-for-measuring-user-interfaces/

Here is a simple pre-launch check you should do on all your web applications. It only takes about 5 seconds and one screeshot

Q: “What percentage of your interface contains stuff that your customers
    want to see?”

1. 10%
2. 25%
3. 100%

If you answer a, or b then you might do well, but you’ll probably get blown out of the water once someone decides to enter the market with option c.