Scheduled value is the result of estimated value minus planned value (SV = EV – PV).
The result for planned value comes from the budget and deadlines set before the start of the execution phase. This means that the estimated value is that factor that will predict if you are ahead or behind schedule. Estimated value is the percent complete multiplied by the original budgeted cost of the task. The only part of these equations that would be altered during execution is the percent complete. Therefore, I agree with the discussion that factors that would affect schedule are task durations. This can be affected by human error or unforeseen setbacks. Proper planning should handle most issues however, there will be some things out of your control as someone mentioned part of a machine being broken. All you would be able to do is try on work on another aspect of the project as you wait for the part to come in. If moving onto another aspect is dependent on the results that are being stalled then your SV would be affected without there being much of a solution.