We're all familiar with usability. Basically it's a way of assessing the success rate of any one task. Successfully finding a book or journal article starting from the home page would be a typical task to measure.
I'm wondering if it might also be helpful to think of task completion in the way marketers do, namely as "conversion rates".
MARKETING 101 FOR LIBRARIES
When librarians use the work "marketing", usually they mean 'getting the word out'. Marketers go one step farther: marketing for them means actually selling a product.