Software Testing: Defect , its cause , cost of finding defect

What is a defect? What are its causes? When do they arise? What is the cost of finding defect?


Defect:-A flaw in a component or system that can cause the component or system to fail to perform its required function, e.g. an incorrect statement or data definition.

Causes of software defects

We know that people make mistakes – we are fallible.

If someone makes an error or mistake in using the software, this may lead directly to a problem – the software is used incorrectly and so does not behave as we expected. However, people also design and build the software and they can make mistakes during the design and build. These mistakes mean that there are flaws in the software itself. These are called defects or sometimes bugs or faults. Remember, the software is not just the code; check the definition of soft-ware again to remind yourself.

When the software code has been built, it is executed and then any defects may cause the system to fail to do what it should do (or do something it shouldn’t), causing a failure. Not all defects result in failures; some stay dormant in the code and we may never notice them.

Defect arises :-In Figure we can see how defects may arise in four requirements for a product.

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 [Jones].

What is the cost of defects?

As well as considering the impact of failures arising from defects we have not found, we need to consider the impact of when we find those defects. The cost of finding and fixing defects rises considerably across the life cycle; think of the old English proverb ‘a stitch in time saves nine’. This means that if you mend a tear in your sleeve now while it is small, it’s easy to mend, but if you leave it, it will get worse and need more stitches to mend it. If an error is made and the consequent defect is detected in the requirements at the specification stage, then it is relatively cheap to find and fix. Similarly if an error is made and the consequent defect detected in the design at the design stage then the design can be corrected and re-issued with relatively little expense. The same applies for construction. however a defect is introduced in the requirement specification and it is not detected until acceptance testing or even once the system has been implemented then it will be much more expensive to fix. This is because rework will be needed in the specification and design before changes can be made in construction; because one defect in the requirements may well propagate into several places in the design and code; and because all the testing work done-to that point will need to be repeated in order to reach the confidence level in the software that we require

1 Comment

Leave a Reply

Your email address will not be published. Required fields are marked *