When Testing should occur?

Testing is sometimes incorrectly thought as an after-the-fact activity; performed after programming is done for a product. Instead, testing should be performed at every development stage of the product .Test data sets must be derived and their correctness and consistency should be monitored throughout the development process. If we divide the lifecycle of software development into "Requirements Analysis", "Design", "Programming/Construction" and "Operation and Maintenance", then testing should accompany each of the above phases. If testing is isolated as a single phase late in the cycle, errors in the problem statement or design may incur exorbitant costs. Not only must the original error be corrected, but the entire structure built upon it must also be changed. Therefore, testing should not be isolated as an inspection activity. Rather testing should be involved throughout the SDLC in order to bring out a quality product.