Verification vs. validation

Testing is the process of finding differences between the expected behavior of the desired system and the observed behavior of the existing system. That is, testing is insuring that what you expect is what you get. You would like to find and fix these differences before your customers find them.

Those differences could be "errors" to be eliminated or "changes" to be made.

More: Errors and changes

You would like to find and fix these differences before your customers find them. Verification involves showing that the implemented solution meets specifications. The solution works right.

verification: the implementation works according to specifications

Validation involves showing that the implemented solution solves the identified problem. The solution does the right work.

validation: the solution solves the identified problem

Question: Compare and contrast verification and validation in terms of a solar-powered flashlight.

Note: You are not told that there is a battery involved. Consider a solar-powered flashlight. You can actually design and implement such a device.

You can verify it.
You cannot validate it, because when the sun is out, you do not need it, and when the sun is not out, it does not provide light.

(Note: Adding a battery is a possible improvement, but, in this case, a battery was not part of the design).

Question: If you are a manager, what is in your best interests, verifying the solution or validating the solution?

To put it another way, to verify a solution is to show that it "works right", while to validate a solution is to show that is does the "right work".

Now for the big question. If you were in management, which would you consider more important, verification or validation? When I ask this question in class, I invariably get the answer, "Why validation, of course. You want to be sure that you have solved the problem." Well, to see the real world perspective, let me rephrase the question. You are the person who is responsible for having just designed and implemented a $100,000,000 system. You can easily verify the solution by showing that you did do what you said you were going to do. Do you want to validate a $100,000,000 system, find out that you did not solve the problem, and then let everyone know? But you can verify that you spent the money the way you said you would. In many cases, when management (or government) proclaims success, but the common worker does not see it that way, it is a success of verification and not validation. At this point, it should be clear why management would prefer to verify and not validate solutions.

Client: "I know that this is what I asked for, but this is not really what I want".

Which part was not completed, verification or validation?

Customers often won't tell you what they really want, particularly if it goes against conventional wisdom. Because they're insecure, they'll tell you instead what they think they're supposed to say they want. James McCarthy (Microsoft software developer and manager)

McCarthy, J. (1995). Dynamics of Software Development. Redmond, WA: Microsoft Press., p. 74.

More: James McCarthy