In my experience, some factors that affect schedule variance negatively involves accounting for unplanned events. Recently we had to complete competitive testing on implants in comparison to our own implants. We had to complete implant characterization but the motors on one of our testing fixtures started stalling. Turns out, the motors were damaged so we had to order two new motors costing about $200 each, which added an additional $400 to the planned budget and also delayed the schedule of completing the competitive testing. It added an additional 2 weeks to complete the testing, which should have been completed in 2-3 days because we had to wait for the motors to come in.