Raluca Budiu is Senior Director, Data Strategy, at Nielsen Norman Group, where she uses her data-analysis expertise to drive strategic decisions. She also serves as editor for the articles published on NNgroup.com. Raluca has coauthored many NN/g reports, as well as the book Mobile Usability. She holds a Ph.D. from Carnegie Mellon University.
In addition to being expensive, collecting usability metrics interferes with the goal of gathering qualitative insights to drive design decisions. As a compromise, you can measure users' ability to complete tasks. Success rates are easy to understand and represent the UX bottom line.