There are many factors that can affect schedule variance (SV) such change requests, issues with suppliers of materials/ components to mention a couple. These factors can impact in the timeline of a project and increase project spending. For all problems there are solutions, as such pre-investing in early phases of the development and ensuring the a well thought out project plan is in place can save time and money in the long run. For example, better evaluating the design during the design phase and having multiple suppliers of materials/components qualified.