Regression vs. Retesting
You must retest fixes to ensure that issues have been resolved before development can progress.
So, retesting is the act of repeating a test to verify that a found defect has been correctly fixed.
Regression testing on the other hand is the act of repeating other tests in 'parallel' areas to ensure that the applied fix or a change of code has not introduced other errors or unexpected behavior.
For example, if an error is detected in a particular file handling routine then it might be corrected
by a simple change of code. If that code, however, is utilised in a number of different places
throughout the software, the effects of such a change could be difficult to anticipate. What appears to be a minor detail could affect a separate module of code elsewhere in the program. A bug fix could in fact be introducing bugs elsewhere.
You would be surprised to learn how common this actually is. In empirical studies it has been estimated that up to 50% of bug fixes actually introduce additional errors in the code. Given this,
it's a wonder that any software project makes its delivery on time.
Better QA processes will reduce this ratio but will never eliminate it. Programmers risk
introducing casual errors every time they place their hands on the keyboard. An inadvertent slip of a key that replaces a full stop with a comma might not be detected for weeks but could have
serious repercussions.
Regression testing attempts to mitigate this problem by assessing the ‘area of impact’ affected by a change or a bug fix to see if it has unintended consequences. It verifies known good behavior after a change.
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