Critical Purpose is a new term to define an approach that emphasises the key deliverables.  On every project there are a small number of critical questions that must be answered if success is to be achieved.  The goal is to isolate these critical success factors and to remove the uncertainties as soon as possible by producing some tangible output that can be dependably validated and indisputably verified. 

Critical Path technique identifies the longest path through a project which defines the shortest possible time to complete the project, according to the estimates of how long each activity will take and the interdependencies between activities.

Critical Chain methodology concentrates on rare resource, thus giving it more of a cost focus.  By planning around the key constraint and reallocating resources, pressure is reduced on bottlenecks.

Critical Purpose will make people think of agile development methods. However, the word agile can conjure up an image of agitation.  The risk is that it over-emphasises reactivity; people may think that if they conceal their intentions, they will be able to make things up as they go along.

Rapid prototyping, risk-based, test-driven, user-centred and team-based are all terms that are used for agile development and each of them emphasises one aspect, whilst in practice the use of one technique makes the others vital. 

If you develop the test plans at the beginning of the project, then you are going to need models and prototypes to test many of the assumptions.  If you build a plan around prototype reviews, then you will need workshops to facilitate the decision making process.  If you focus on risks, then you are going to need the presence of the team and subject matter experts.  If you prioritize the requirments then you need to involve users.  In fact, of course, you need all of these.  It is reassuring to think that if you do one of them, it pulls the others.

‘Critical purpose’ is a term that covers all of these approaches, at least as well as agile because it does not lead people to think that they can decide everything at the last minute, or on their own on their way to work, or without informing anyone of their reasoning.