Metrics are an important thing to get right (particularly when you're looking to demonstrate ongoing improvement), and Jeff's article suggests nine beyond the basic success or failure rate.
Working with stakeholders ahead of your research to agree what will be measured and what is considered to be an acceptable figure is a great way to engage others in the process and foster buy-in for your work.
Jeff's "essential usability metrics":
- Completion Rates
- Usability Problems
- Task Time
- Task Level Satisfaction
- Test Level Satisfaction
- Page Views/Clicks
- Single Usability Metric (Combining a range of metrics into a single score)
I've never considered using a single usability metric before, but think this could be just the thing I need to help me express how we're doing in a report card-style summary. (I wrote about Lou Rosenfeld's report summary in October).
10 Essential Usability Metrics - article by Jeff Sauro