In addition to the factors mentioned by msc52njit-edu that can negatively impact the SV of a project, from my experience working on medical device projects one of the factors which often had a negative impact on SV was supplier delays. Since we outsource all our manufacturing, we depend quite heavily on our suppliers to develop their own timeline for manufacturing our devices. However, sometimes they make unrealistic assumptions and their lead time ends up being much longer than they estimated originally. There are several ways to reduce the possibility of supplier delays. It is important to set expectations early on so the supplier knows that a delay is unacceptable. Possibly charging a fee for late delivery could also motivate the supplier to provide realistic lead times and to deliver on time. Also, it is very important to maintain clear lines of communication and transparency between you and the supplier as this will reduce the possibility of the timeline slipping.