Project Management hard skills, such as Costing, Planning, Scheduling, Scoping, and Managing Risk, are critical for any Project Manager. However, these skills are only useful during specific phases in the project. Or, they may come into use on multiple occasions. For example, a Project Manager will perform the costing of a project once, but will keep wearing the hat of a Cost Manager throughout the project to ensure the cost performance index hovers around "1". Alternatively, the Project Manager will create a project network diagram once, but may revisit it and make changes. Consequently, the critical path of the project may also change.