Best practice is a method or technique that has consistently shown results superior to those achieved with other means, and that is used as a benchmark. In addition, a “best” practice can evolve to become better as improvements are discovered. Best practices are used to maintain quality as an alternative to mandatory legislated standards and can be based on self-assessment or benchmarking. (Adopted from Wikipedia).
Methodologies are a set or system of methods, principles, and rules for regulating a process or an action, while guidelines are statement by which to determine a course of action. A guideline aims to streamline particular processes according to a set routine or sound practice. By definition, following a guideline is never mandatory. Guidelines are not binding and are not enforced.

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