According to the PMBOK® Guide, specifically within the Develop Schedule process, Schedule Compression techniques are used to shorten the project schedule duration without reducing the project scope. There are two primary techniques recognized by PMI: Crashing and Fast Tracking.

Crashing is a technique used to shorten the schedule duration for the least incremental cost by adding resources.

Adding Resources: Examples include approving overtime, bringing in additional vendors, or adding more team members to activities on the critical path.

Cost-Schedule Trade-off: Crashing almost always results in increased costs. It is only effective for activities on the Critical Path where additional resources will actually shorten the duration.

Risk: While it shortens the timeline, it may increase risk or lead to diminishing returns if too many resources are added to a single task (the Law of Diminishing Returns).

A. Activity sequencing: This is the process of identifying and documenting relationships among the project activities. It defines the logical order of work but is not a technique used to " compress " or shorten an established duration.

B. Resource leveling: This is a resource optimization technique in which start and finish dates are adjusted based on resource constraints. Resource leveling often causes the original critical path to increase (lengthen), which is the opposite of compression.

C. Lead and lag adjusting: While adjusting leads (advancing an activity) can sometimes help overlapping, " Lead and lag adjusting " is a general refinement of dependencies. Fast Tracking is the specific compression technique that involves overlapping phases or activities that are normally done in sequence.

Crashing: Adds resources to shorten duration. Increases Cost.

Fast Tracking: Performs activities in parallel that were originally planned in sequence. Increases Risk.