In the original Ghostbusters film, Bill Murray tells Sigourney Weaver that he never sleeps with possessed women. She then gives him a kiss at which he says “Actually, it’s more of a guideline than a rule”. It’s my favorite definition of the difference between a guideline and a rule and it is the difference between ITIL® and ISO / IEC 20000.
ITIL is a best practice framework. From its earliest days, “Adopt and adapt” has been the implementation mantra. In essence, you can’t really “implement” ITIL. You can implement an ITIL based approach but ITIL provides pretty high level, process advice and guidance. You have to fill in the detail of how the processes will work in practice in the context of your own organization. Often this will mean some degree of modifying the ITIL process models to work with the way you currently do things rather than the other way round – if it ain’t broke don’t fix it.
On the other hand, ISO/IEC 20000 is a standard. Sure, you can use ISO/IEC 20000 as an adaptable framework in the same way that you would use ITIL. It provides a great mechanism for internal benchmarking. But if you are seeking external accreditation against ISO/IEC 20000 then Part 1 of the standard, which defines the “must haves”, are rules not guidelines. They are not flexible and outline the minimum requirements for your service management processes.
And don’t try kissing your external auditors – this isn’t the movies!