In a situation where a software product is under development, regression testing is essential in discovering (and fixing) new defects introduced during coding, usually as a result of adding or fixing code.

Ideally, every change in the product, regardless of how minor, would be verified by thoroughly and rigorously re-testing the whole product to ensure no new bug has been inadvertently introduced and that the requirements specification is still met. Automated testing, with little or no human intervention, is usually the only economically feasible approach to take with regression testing.

retesting of a previously tested program following modification to ensure that faults have not been introduced or uncovered as a result of the changes made, and that the modified system still meets its requirements. It is performed whenever the software or its environment have changed.
BS7925-1

In other words, running the test scripts that succeeded before to see if they still do.

Usually contrasted with retesting, which is running the scripts whose failure pointed you toward the bugs in the first place.

Regression testing also occurs when you put something through a test so that you can create a graph that has something proportional to something. You can draw on a line of regression that predicts future outcomes. The line of regression is another word for the line of best fit on a graph. When you create outcomes you can interpolate the data and judge what will happen if something is extended.

For example, if you have a car and put the speed travelled along the Y axis and time taken along the X axis. You can see how long it takes to get up to a certain speed. However if the car cannot go any faster then you will need to use the line of regression to predict how long the car will take to travel at the unreachable speed. This can be useful for example when a selling a car. Most car companies give technical information such as “zero to sixty miles per hour in nine seconds.” What if the car’s top speed is 50 miles per hour? Well we can use the line of regression to predict the time it takes if it could have reached 60 miles per hour.

Therefore, the line of regression and regression testing plays a very important role in working out future events in the real world. We need it to find out the outcome of something that hasn’t happened before.

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