Group decision-making techniques are some of the tools and techniques used in the PMI processes (estimate activity durations, validate scope, collect requirements, and estimate costs). Some decision techniques are unanimity, majority, plurality, points allocation, and dictatorship. For unanimity, everyone must agree; there is a shared consensus. One technique suggested in the PMBOK is the Delphi method. A majority or plurality is usually determined by a vote. For a majority, the decision must be agreed to by more than half the participants. In a plurality the option with the most votes wins, even if fewer than half the participants chose it. With points allocation each voter is given a number of points. Then the voter allocates points to the options. For instance, if each voter gets three points, one voter could give all three points to option A, while another voter gives option A one point and two points to option C. The option with the most points is chosen. (Note: points allocation is not one of the techniques listed in the PMBOK.) In a dictatorship, one person makes the decision.
The project planning documents should identify how major decisions will be made. A combination of methods may be used. Different techniques maybe used for decisions in different PMI knowledge areas or at various levels of importance or potential financial impacts. For instance, the team may make a decision recommendation based on points allocation. Then the project manager might have permission to approve those decisions as long as they do not change the projected cost by more than 5%, the timeline by more than 10% or the approved scope. For decisions outside of that range, the sponsor's approval could be required (in this example).
See the PMBOK 5th edition, section 188.8.131.52.