While you and your pair partner are working on a task, you run unit tests to make sure you are not introducing side
effect defects. As a system evolves, it may become impractical to run all the unit tests for the system for every
change made. In this case, you may choose a subset of the unit tests to run after every code change. When a task is
completed or you feel that you have a piece you can integrate or you are uncertain for any reason, run all of the unit
tests for the system.