I believe that the actual purchasing of raw materials or even contacting with prospective vendors is best carried out while under the execution phase of a project. However, I can see as to why some may feel that it’s best carried out while under the planning phase. Source selection during the planning phase does have its pros as it does pull the project ahead of schedule and may even jump-start the execution of the project earlier than expected. However, that itself can be exactly why it can work as a double edged sword. If a project team is already going off purchasing raw materials from said vendor when the project is still in its planning phase then things can go wrong as in changes to design, specifications, inputs etc. for example. It is best to hold off on spending company resources until documents such as the DSD and DDP start to become more solidified. What I think is best during the planning phase is for project managers to only start thinking about potential vendors so that they can be prepared for actual purchasing and price comparisons during the executing phase.