As mentioned above most have spoken on factors which have a negative outcome on the SV. However, there are positive factors which lead to a positive SV. One of those factors is to use the DHF (Design History File) . A company should have this in their collection of documents. This helps to eliminate errors which have been made in the past. The errors may not just be scheduling issues but, can help in other areas of the project. Anyway, say a DHF has a gnatt chart from a previous project and it had comments as to what impeded the progression of the project, those insightful comments, can help with the projects SV. Also, this can be considered Risk Management what I am posing here as well. Having a risky strategy if a difficult do arise may be bypassed because there is an alternative or fix to the issue currently taking place.