Forced degradation studies are used to identify reactions which may occur to degrade a processed product.  Usually conducted before final formulation, forced degradation uses external stresses to rapidly screen material stabilities.
Longer-term storage tests are usually used to measure similar properties when final formulations are involved because of the stringent FDA regulations.  These tests are generally more expensive (because of the time involved) than forced degradation which is therefore used for rapid selection and elimination tests.