As @cdj24 mentioned SV=EV-PV. Where EV is the percent complete* original budgeted cost of task and PV is the vale of the work you are supposed to have on a certain date. I agree as others have mentioned that in order to have a positive SV which means you are ahead of schedule, you need to have have a larger percent complete than planned. The factors that would contribute to the success or downfall of an SV would include as others have mentioned; resources including being understaffed/vacation and other unplanned setbacks that would affect the schedule.
I personally have seen that when we contract out a medical device the company will create their gantt chart and timeline, but in order for them to move forward to next critical task they will need sign off from our PI. If it is a surgeon, surgeons are very busy and for larger robotic devices, it usually requires the surgeon to go to in-person to test the device and this can cause setbacks in the timeline due to the busy schedule of the surgeon. Sometimes the contracted party isn’t even located in NY so it can really be difficult to make some critical design decisions unless the surgeon is able to test it in person. In some situations the PI wants to then change an aspect of the deign or had another idea and this could add not only to the Schedule Variance but also the Cost Variance. CV= EV=AC (Actual cost). Unfortunately making these changes means that we go over budget. As a solution, we try and address this early on to work with surgeons admin on the critical design/testing milestones meetings to stay in positive SV and educating the surgeons on the importance of spending time on surgeon/user needs upfront so that we don’t get behind schedule or over budget re-designing the device, which can be costly and result in us going over budget. Even with these best practices and intentions, unplanned circumstances happen and can still result in a negative SV and CV. I think good planning and communication among all parties involved is really the key to have a positive SV and CV.