If a project's Schedule Performance Index (SPI) is 0.9, what specific schedule status does this immediately convey to an expert?

An SPI of 0.9 immediately conveys that the project is behind schedule. The Schedule Performance Index (SPI) is a key metric in Earned Value Management, calculated as Earned Value (EV) divided by Planned Value (PV). Earned Value (EV) represents the budgeted cost of the work actually performed to date. Planned Value (PV), also known as Budgeted Cost of Work Scheduled (BCWS), is the budgeted cost of the work that was scheduled to be completed by the measurement date. When the SPI is 0.9, it means that for every dollar's worth of work planned to be completed by a specific point in time (PV), only ninety cents' worth of work has actually been accomplished (EV). Since the Earned Value is less than the Planned Value (EV < PV), less work has been completed than was scheduled, directly indicating a behind-schedule status.