I could not say better than Alistair Cockburn in Crystal Clear Applied: The Seven Properties of Running an Agile Project: (excerpt)

I don't believe that any prescribed procedures exists that can assure that projects land in the safety zone every time. Nor, with the exception of incremental development, do I show up on a project with any particular set of rules in hand, even though I have my favorites. This is why Crystal Clear is built around critical properties instead of specification of procedures.

Signatory of Agile manifesto