Impact analysis – when and how does it improve quality

Impact analysis evaluates the changes that were made to a system/application to identify the intended consequences as well as possible side effects of a change and to identify the areas in the system that will be affected by the change.
This means that impact analysis should be done before a change is made to help decide if the change should be made.
Impact analysis help improve the quality of the software by reducing the likelihood of finding unexpected defects i.e. this would be considered as a defect prevention technique.