Explain how can the schedule performance index(SPI) Be used in schedule monitoring and control?

Schedule performance index SPI = BCWP/BCWS (budgeted cost for work done over budgeted cost for work planned to be done) Normally the value remains less than one.
But if you execute more value of work than that was scheduled during a particular period then you have earned more than planned costs

When the SPI <1 You are behind the schedule.
Take appropriate measures, create remedial action plan when SP =1 You are as per schedule.
When SP >1 You are performing better and completing the task earlier than planned.