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I have always thought that “descriptive” models says “what” but do not specify “how”, while “prescriptive” ones do exactly that. But when I started checking the topic, I found a lot of contradictions. Some say that process reference model in ISO 12207 is not prescriptive, while others say it is. As common examples of prescriptive models, the following are being named: waterfall, incremental, spiral… but to me it does not make sense, those are much more vague than the aforementioned ISO. CMM is said to be descriptive, yet to me it seems much more specific than e.g. the waterfall (I acknowledge these are dealing with different topic but I focus on the level of details).

he most common dispute I can find about ISO 9000. Some sources says it prescriptive, some say its descriptive.

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    I would say ISO 9000 is prescriptive in its descriptivism. It requires (i.e. prescribes) companies to document (i.e. describe) how they intend to meet its quality management principles and what has actually been done, but it does not impose a single detailed model on different companies. – Henry Aug 7 '16 at 11:33
  • @Henry Yes, I would assume the same. A simple check in Google books, some claim ISO 9001 is clearly descriptive as it only says "what" but not "how", others claim that those requirements are actually prescribing so ISO 9001 is prescriptive. I am puzzled how something like that is still a subject of subjective judgement.. – John V Aug 7 '16 at 15:39
  • This is interesting. If looking at the use of descriptive vs. prescriptive in regards of grammar , which for some reason filled my google results, it's not "what" vs. "how", but rather "is" vs. "should". Is it not the same in this domain? – Alex Aug 7 '16 at 20:58
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    Your understanding of these terms is almost certainly wrong. Prescriptive means "This is a rule." Descriptive means "describing or classifying without expressing feelings or judging," or "not a rule." – Robert Harvey Aug 8 '16 at 2:31
  • @RobertHarvey No, there are different context for these terms, however in terms of SW modeling, the common (mentioned also in books) definition is that prescriptive is normative, telling exactly how to proceed, while descriptive is just describing the properties of the result, goal but not specifying the way to get there. – John V Aug 8 '16 at 11:35
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"Prescriptive" means that they tell you what to do ("relating to the imposition or enforcement of a rule or method.") "Descriptive" means they describe something that can be done ("serving or seeking to describe.") It has nothing to do with vagueness or detail.

The ISO 12207 standard sets forth names and definitions for various software activities, and describes how they can be of benefit. Meanwhile, waterfall, incremental, and spiral tell you specifically how to run your project.

For example,. ISO 12207 lists several general categories of activities: Acquisition, Supply, Development, Operation, Maintenance, and Destruction. These are general enough to apply to any software project, and indeed, you can find articles on how to apply ISO 12207 to Scrum or other methodologies. You certainly couldn't apply Waterfall to Scrum!

Meanwhile, a process like waterfall is very specific-- it not only lists phases, but tells you what order to do them in, and is very clear that failing to finish one phase before moving to the next is wrong. This is prescriptive.

ISO 9000 is pretty much descriptive as well, but an organization can become "ISO 9000 certified," meaning that they have chosen to comply with the standard rigorously. At that point it becomes prescriptive.

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It's all in how you use it. The point is to document a process. This can show how an intent is expressed. If two parties conform then intent is expressed as expected. Otherwise someone gets surprised. Your question centers around who's problem that is. Either change the expression, the standard (yet another version), or let the standard become nothing more than a suggestion.

It is certainly decriptive. Wether it's proscriptive depends entirely on who your working with. If it's not in the contract it's just something to read when trying to get to sleep.

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