A project can be broken down into segments with each subproject being used to develop a subnetwork diagram of activities. This technique of dividing the project into sections can make it more manageable for the project management team. As a result the project can be reported at various summarization points throughout the course of project management.
A summary activity or hammock activity groups related activities in a schedule over a segment of the project’s life. By using hammock activities the summarization process for the project is simplified by minimizing the total number of activities to be displayed.
Hammock activities are generally used to report information that is time-dependent and lasts from the earliest to the latest date of the activities it encompasses. The duration of the activity is normally determined following the drawing up of the network plan. Using a hammock activity allows top management to see an overview of the project without being overwhelmed by details.
A summary activity is usually considered to be an overhead cost since project resources are typically used. In addition, most project management software is designed to calculate the duration of the linked summary activity.

This term is defined in the 3rd and the 4th edition of the PMBOK.

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