Roles incompatibles: Project Manager y ScrumMaster

En muchas empresas de desarrollo de software, se asigna en el rol de ScrumMaster a un Project Manager, al mismo tiempo en el mismo proyecto. A mí me paso y pensaba que era lo correcto, que tenía sentido, hasta que me encontré con un artículo que me voló la cabeza. Citando a Steven Hundon: 

Contrary to popular belief, the ScrumMaster and project manager roles are highly different and shouldn't be confused. As more companies migrate their project management to Agile, many do so without a proper understanding of what they're aiming for. In particular, there are incorrect assumptions made about the roles in Agile; people often expect that the shift from Waterfall practices includes a wholesale shift of roles. The ScrumMaster, however, does not play the part of the traditional project manager. Traditionally, the project manager is a leader, a decision maker, a planner, someone who manages the project and the team and is the person accountable to the business for accomplishing the project objectives. The ScrumMaster's role is more that of coach and facilitator, a role that sits between the project and the customer. The ScrumMaster doesn't manage the team that produces the work; instead, he supports the product owner, coaches the team, and makes sure that Scrum processes are adhered to. The ScrumMaster is responsible for the Scrum process, its correct and continuous implementation, and the maximization of its benefits.

Esto realmente tiene todo el sentido del mundo. En Scrum, se distingue la persona que da soporte al equipo en QUE hace y COMO lo hace, mediante el Product Owner y el ScrumMaster respectivamente, siendo dos personas distintas con objetivos distintos. Veamos el siguiente diagrama:

¿Porque el ScrumMaster y el Project Manager no pueden ser la misma persona? En Scrum no se le puede pedir a alguien que sea responsable del QUE y del COMO a la misma vez porque bajo presión, se va a favorecer a uno u a otro. Si el ScrumMaster toma decisiones sobre el producto, Scrum no está correctamente implementado ya que da lugar a conflictos y confusiones.