Stephen Devaux's book Total Project Control introduced a set of new techniques and metrics on the basis of return on investment (ROI) and expected monetary value (EMV). They include value breakdown structure, critical path drag and drag cost. The concept of drag is an extension of the Critical Path Method (see above) - activities that lie off the critical path are said to have "float", while those on the path have "drag". Importantly, the drag of any critical path activity can be calculated (if there are no parallel activities the drag is equal to the duration; if there are parallel activities the drag is equal to the the lesser of its total duration or the total float of the parallel activity.) Drag calculations allow project managers to more easily prioritise activities.