Cell phone screen covers are either flexible plastic or hard plastic. An experiment is to be conducted to determine whether the swiping speed is the same for both types of screen covers. In previous studies, it was determined that cell phone size (small, medium, large) is associated with swiping speed, but cell phone type (Android, iPhone, and so on) is not associated with swiping speed. How could the experiment BEST be conducted?
By blocking on swiping speed
By blocking on screen cover type
By blocking on cell phone size
By blocking on cell phone type
Without blocking
