Which method assigns a single fixed duration to each activity in a project?

The method that assigns a single fixed duration to each activity in a project is Critical Path Method (CPM). CPM is primarily used for projects where the duration of individual tasks is predictable and can be estimated with a high degree of certainty. This method focuses on determining the longest stretch of dependent activities and measuring the time required to complete them from start to finish. By establishing these fixed durations, project managers can identify the critical path, which is the sequence of activities that determines the project's overall duration.

In contrast, Program Evaluation and Review Technique (PERT) allows for variable durations and emphasizes estimating the time needed to complete each task based on a probabilistic approach, which involves optimistic, pessimistic, and most likely estimates. Resource leveling, on the other hand, is aimed at optimizing resource allocation over a project's duration but does not inherently define task durations. Lastly, time boxing is a technique to allocate a fixed time interval for a certain task or activity but does not apply a fixed duration to each activity in a traditional project management sense.