Qualitative usability testing aims to identify issues in an interface, while quantitative usability testing is meant to provide metrics that capture the behavior of your whole user population.
Raluca Budiu is Director of Research at Nielsen Norman Group, where she consults for clients from a variety of industries and presents tutorials on mobile usability, designing interfaces for multiple devices, quantitative usability methods, cognitive psychology for designers, and principles of human-computer interaction. She also serves as editor for the articles published on NNgroup.com. Raluca coauthored the NN/g reports on tablet usability, mobile usability, iPad usability, and the usability of children's websites, as well as the book Mobile Usability. She holds a Ph.D. from Carnegie Mellon University.