The declining schedule performance index (SPI) indicates that the project is not progressing as planned. The first step should be to understand the root cause of the deviation and to adjust the project baseline accordingly if necessary. This is in line with the PMBOK guide's principles, which suggest that when project performance deviates from the plan, the project manager should review and analyze the project schedule to understand the reasons behind the deviation and to manage stakeholder expectations1. Rebaselining the project may involve redefining the schedule, scope, and cost baselines to reflect the current understanding of the project. This approach allows the project manager to set a more realistic path forward and to communicate changes to stakeholders effectively.
References: The PMBOKGuide and the PMP Examination Content Outline provide guidance on the project manager's role in monitoring and controlling project work, which includes managing schedule deviations and rebaselining as necessary2. The Professional in Business Analysis Reference Materials also emphasize the importance of accurate and realistic project planning and the need for continuous monitoring and adjustment3.