Hammock Activity

A hammock activity — sometimes called a summary or span activity — is a special type of schedule element that has no independent duration or logic of its own. Its start is tied to the start of the first activity it spans, and its finish is tied to the finish of the last activity it spans. The result is an activity that automatically reflects the current status of a group of activities below it. Hammocks are used to create high-level summary views of a detailed programme, to represent project phases in a management-level Gantt, and to track costs or resources against a group of activities as if they were a single block.

Hammock activities are particularly useful for reporting to stakeholders who need a simplified view of the programme. Rather than showing a hundred detailed activities across a commissioning phase, a single commissioning hammock shows the earliest start and latest finish of that entire block of work. When the underlying activities change — because of delays, acceleration, or re-sequencing — the hammock automatically adjusts to reflect the new boundaries. This makes hammocks a much more reliable summary mechanism than manually maintained summary bars, which become stale the moment the detail below them changes.

The key distinction between a hammock and an LOE activity is that a hammock is driven entirely by the dates of its subactivities, while an LOE activity is driven by its own duration (which is set to match a supporting relationship). Hammocks are also different from WBS summary bars in some scheduling tools, which may or may not behave the same way depending on how the tool calculates summary-level dates. In Primavera P6, for example, WBS summary activities and hammock activities behave differently and are used for different purposes. Understanding which type you are working with is important when interpreting summary-level critical path information.