... key project players have been hired or otherwise brought into the project and cover the necessary expertise (DomainExpertiseInRoles) but the project needs more staff.
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Growing projects must figure out how to grow long-term staff: whom to hire, how many to hire, and when to hire them. Projects must ramp up while minimizing the pains of growth.
You need enough people for critical mass. Yet you cannot just hire anyone off the street; staff are not plug compatible and interchangeable.
The right set of initial people (SizeTheOrganization) sets the tone for the project, and it's important to hire the key people first. You need a critical mass of key people early on. Yet too many people too early create a burden for the core team.
Therefore:
Phase the hiring program. Start by hiring people to meet the basic core competencies of the business and gradually bring on new people as the project needs to grow.
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The organization can staff up to meet development load. This pattern is closely related to ApprenticeShip and to ModerateTruckNumber. DayCare can be applied to help with the training and mentoring load that new employees place on the organization.
This is a well-known management technique that allows the project to establish an identity early on, and to grow graciously.
Larry Putnam points out that projects that grow very quickly at the beginning tend to be late. He advocates growing staff gradually. [BibRef-Putnam1992].
In The Mythical Man-Month Brooks states, "V. A. Vyssotsky of Bell Telephone Laboratories estimates that a large project can sustain a manpower buildup of 30 percent per year. More than that strains and even inhibits the evolution of the essential informal structure and its communication pathways." [BibRef-Brooks1995], page 293.
What constitutes "core competencies?" Part of this depends on the business you are in. If you are in finance, you want people who can develop financial software. The better people you can get early on, the better off you will be, and it is probably a good return on investment to spare no expense on talent at this early stage. Talent isn't limited to domain knowledge, though; you also need individuals who can put customers at ease, who can keep a cool head for strategic planning, who can "fill in the cracks" by doing the miscellaneous detailed tasks that others don't want to do or forget to do, etc. Many individuals have many of these talents; the key is to cover the crucial needs early on with as few people as possible, and to grow the organization once that organization has gelled (see StableRoles). You can achieve these goals with HolisticDiversity and DiverseGroups.