...the project is incrementally staffing up after the first round of experts have been brought on board.
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A project must balance its need for growth with its need to develop and maintain deep domain expertise. You need enough people for critical mass. However, staff are not plug compatible and interchangeable. And academic training and prior experience are rarely, in themselves, adequate preparation for competent work at a new task.
Therefore:
Turn new hires into experts (see DomainExpertiseInRoles) through an apprenticeship program. Every new employee should work as an apprentice (not just a mentee) to an established expert. Most apprenticeship programs will last six months to a year--the amount of time it takes to make a paradigm shift.
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It will be possible to maintain expertise in the organization. This pattern also reduces the organization's "truck number" (the smallest number of people such that, if any one of them were hit by a truck, the organization will have lost a critical resource; see ModerateTruckNumber) by spreading knowledge around. The "masters" feel valued and the apprentices are given a good environment to learn.
Manage drain on expert staff resources with DayCare.
DevelopingInPairs is often used as an effective ApprenticeShip technique.
It is better to apprentice people than to put people through a "trial by fire" that may damage the project. The apprenticeship approach makes it possible to form domain-specific teams, and it is important to keep the team concept as a central part of organizational values.