Prescriptive and descriptive models of SW development - why it is not clear?

Question

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.

Answer 1

"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.

Answer 2

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.