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Some background. Functional requirement of a web service is concerned with the correctness of the web service's function — say, the service will always return a number that is less than two.

The non-functional requirement of a web service is concerned with the performance of the web service — say, the service will respond in 5 ms.

Now, given the phrase:

The mixed functional and nonfunctional requirement concerns both — say, the service will return a number that is less than two in 5 ms.

Is "the mixed functional and nonfunctional requirement" grammatical?

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I'm not sure about mixed. How about "The combined functional and non-functional requirements..."? – Kristina Lopez May 2 '13 at 17:33
or why not simply: functional and nonfunctional requirements. Though just because the customer has asked for something that you think is a non functional requirement does not mean it can become a function/ requirement. I would probably not use a slow responding website (part of the reason for the popularity of facebook and stack sites over other forums) – tgkprog May 2 '13 at 17:37
there are pure non functional requirements. For example : 1. logs, 2 modular design that allows changes to be done relatively easily. – tgkprog May 2 '13 at 17:38
Voted to close as off-topic, since the question was also posted verbatim on ELL. – Hellion May 2 '13 at 21:10

I would say that rather than spelling out "mixed functional and non-functional" all the time, you should define it once and then go with the defined term:

A functional requirement is concerned with the correctness of the service's function...

A non-functional requirement is concerned with the performance of the service....

A mixed requirement specifies both functional and non-functional elements, for example that a call to the service will return a value less than two within 5 ms.

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Why conflate the requirements?

Functional requirement are about what the system does.

Non-functional requirements are about how the system achieves the functional requirements - generally constraints that the system is bound by.

The general method is to list the functional requirements. Then list the non-functional requirements. Keep them split apart.

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