The selection process for projects is explained in Section 5.1 of the ALMA Cycle 2 Proposers Guide:
"Science observations will be executed by ALMA operations staff, taking into account (in rough order of priority): the weather conditions, the configuration of the array, target elevation and other practical constraints, the projects’ assigned priority group, and executive balance. All other things being equal, the project with the highest scientific rank will be observed."
Proposals will be assessed on the overall scientific merit of the proposed investigation and its potential contribution to the advancement of scientific knowledge. For a full description of the proposal selection process, see Section 7 of the ALMA Cycle 2 Proposers Guide.