A regular study to measure against a baseline

The following are goals that we have encountered when UX studies were initiated to measure the evolution of the product's usability against a baseline: 

• "We've implemented all of the recommendations from the previous study to improve the usability, and we now want to determine whether the users appreciate the changes."

• "It's time for our quarterly usability study."

Ideally, each usability study will be a part of a series of studies, where the first study sets the baseline, and all subsequent studies validate the improvements made to the product, and will hopefully measure increased usability in whatever form it was measured originally (task completion rates, satisfaction scores, and so on).