Selecting a Scrum Master
Which should be the best way to select a Scrum Master? As usual ... You should pick someone that could match with the responsibilities and skills required by this role.
Right... And how we will identify these responsibilities and skills?
We can start with the description of these responsibilities from the Scrum Guide and from the official books and papers of the Scrum authors.
Suppose that we have this scenario: the criteria selection is to pick someone that have "Scrum Master" certification. A such candidate will know the Scrum rules, and there is an exam result to prove that (and possible also a Scrum course).
It is that a good criteria?
I think that is not enough.
First - most of the certifications are CSM (from Scrum Alliance) and then PSM 1 (from Scrum.org). The problem is that these certifications are "entry-level" certifications for both organizations. There are also more advanced certifications in both cases: CSP, CSC (from Scrum Alliance) and PSM 2 (from Scrum.org). The highest level certifications are for the official trainers: CST and PST. The problem is that there are fewer people with these kind of certifications and it is more expensive to use such professionals.
Second - It is not enough to know only the Scrum rules. The SM must have Agile and process experience (per domain - such software development) in order to offer guidance for both PO and DT.
Jeff Sutherland says that it was inspired to create this role by the work of Taiichi Ono to set up the first Toyota Production System team "a facilitative leader" that could help "cross-functional team do their work.". Later the SM evolves more to the coaching dimension.
Jeff Sutherland conclusion: "..the Scrum Master is responsible for significantly improving performance of the team both in terms of output and happiness. So you can't just pick anyone for the Scrum Master and I have never seen a great team without a great Scrum Master."