Re-testing and regression testing are essential concepts in software testing, each serving a different purpose.
According to the ISTQB CTFL Syllabus, version 4.0, the difference between these two types of testing is highlighted as follows:
* Re-testing(also known as confirmation testing) involves executing the same tests again on the same version of the software after defects have been fixed. The primary goal is to confirm that the original defects have been successfully removed and that the software behaves as expected.
* Regression testing, on the other hand, is performed to ensure that recent code changes have not adversely affected the existing functionalities of the software. This testing aims to detect any unexpected side effects introduced by the new changes.
The syllabus states: "Re-testing is running a test again; regression testing looks for unexpected side effects".
This explanation clarifies that while re-testing focuses on verifying specific defect fixes, regression testing ensures the overall stability and integrity of the software after changes.

When what is visible to end-users is a deviation from the specific or expected behavior, it is called a failure. A failure occurs when the software does not perform a function as expected or produces incorrect results, which can be observed by the users. This is distinct from an error, which is a human action that produces an incorrect result, a defect (or bug), which is a flaw in the software, or a fault, which is another term for a defect.

A valid objective of testing is to prevent defects. While correcting defects and locating defects in the code are part of the testing process, the primary goal is often to prevent defects by identifying issues early in the development cycle and improving the overall quality of the software. Ensuring no defects are present is an unrealistic objective, as testing can demonstrate the presence of defects but not their absence.
Reference: ISTQB CTFL Syllabus, version 4.0, emphasizes the objectives of testing, including defect prevention as a key goal.

Equivalence partitioning divides input data into partitions where each partition is expected to be treated the same way by the system. The values $2,000 (no duty), $2,001 (taxed at 10%), and $10,000 (partially taxed at
12%) fall into three different equivalence classes based on the provided tax brackets, ensuring different behaviors for each class.
Reference: ISTQB CTFL Syllabus, version 4.0, describes equivalence partitioning and its application in defining different classes of input data for testing .

A disadvantage of independent testing is that developers might lose a sense of responsibility for the quality of their work. When developers rely too much on independent testers to find defects, they may become less diligent in their own testing and quality assurance efforts. The ISTQB CTFL Syllabus pointsout that while independent testing can provide an unbiased perspective and identify defects that might be missed by developers, it can also lead to potential downsides such as diminished developer accountability.