The critical path of a project is the longest chain of non-parallelizable tasks in terms of total time taken for completion. Other tasks may have dependencies, prerequisites, or exhibit 'total float', which indicates that they can occur at any time without delaying the rest of the project. Tasks on the critical path, however, will likely determine the overall flow of the project. The critical path can therefore be regarded as an upstream constraint. Related to the ideas of rapid prototyping and key assumption analysis, once the critical path has been identified a cheap way to test the 'weakest link' in the chain is desirable.