Explanation This is an example of a corrective action, which is an action taken to bring the project performance back in line with the plan. The schedule performance index (SPI) is a measure of schedule efficiency, expressed as the ratio of earned value to planned value. An SPI of less than 1 indicates that the project is behind schedule. In this case, the SPI was 0.87, which means that the project was only 87% as far along as it should have been at milestone 6. To correct this situation, three project team members worked overtime to catch up with the planned schedule. This is a corrective action because it addresses an existing problem and aims to eliminate the root cause of the deviation. References: Schedule Performance Index (SPI) for PMP Exam Prep, CBAP Training and Exam Preparation - IIBA Endorsed Course, CBAP Step-by-Step Application Process Guide