What does a Schedule Performance Index (SPI) value less than 1 indicate?

A Schedule Performance Index (SPI) is a measure used in project management to assess the efficiency of time utilization on a project. The SPI is calculated by dividing the earned value (the value of the work actually completed) by the planned value (the value of the work that was scheduled to be completed by that time).

When the SPI value is less than 1, it signifies that the project is not performing as scheduled; specifically, it indicates that the earned value is less than the planned value. This means that the actual progress of the project is lagging behind what was planned, which is why it is interpreted as being behind schedule. Understanding this metric is critical for project managers as it helps them identify timelines that are not being met and allows them to take corrective action to bring the project back on track.