During which project management process group is rough order of magnitude estimate made?

A rough order of magnitude (ROM) estimate takes place very early in a project’s life cycle — during the project selection and approval period and prior to project initiation in most cases.  The main purpose of the ROM estimate is to provide decision-makers with the information necessary to make a decision on whether it makes sense to move forward with the project based on the estimated level of effort, in terms of completion time and cost.