Because of the following reasons the software defects arise:

• The person using the software application or product may not have enough knowledge of the product.
• Maybe the software is used in the wrong way which leads to the defects or failures.
• The developers may have coded incorrectly and there can be defects present in the design.
• Incorrect setup of the testing environments.

To know when defects in software testing arise, let us take a small example with a diagram as given below.

We can see that Requirement 1 is implemented correctly – we understood the customer’s requirement, designed correctly to meet that requirement, built correctly to meet the design, and so deliver that requirement with the right attributes: functionally, it does what it is supposed to do and it also has the right non-functional attributes, so it is fast enough, easy to understand and so on.

With the other requirements, errors have been made at different stages. Requirement 2 is fine until the software is coded, when we make some mistakes and introduce defects. Probably, these are easily spotted and corrected during testing, because we can see the product does not meet its design specification.

The defects introduced in Requirement 3 are harder to deal with; we built exactly what we were told to but unfortunately the designer made some mistakes so there are defects in the design. Unless we check against the requirements definition, we will not spot those defects during testing. When we do notice them they will be hard to fix because design changes will be required.

The defects in Requirement 4 were introduced during the definition of the requirements; the   product has been designed and built to meet that flawed requirements definition. If we test the product meets its requirements and design, it will pass its tests but may be rejected by the user or customer. Defects reported by the customer in acceptance test or live use can be very costly. Unfortunately, requirements and design defects are not rare; assessments of thousands of projects have shown that defects introduced during requirements and design make up close to half of the total number of defects.

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