Extensive testing means testing an application thoroughly.

NEED : Envolvement of High Risk In implementation
Amount of testing to be performed on a component/system 
will be determined by the risk associated with placing the 
software into operation. If the risk is minimal,a  decision 
may be made not to test. If the risk is high, extensive 
testing may be required.

Actor : This is done by testers(independent) because 
developers are optimistic in their ability to do defect-
free work and thus underestimate the need of extensive 
testing.