Many Project Managers use the terms Hammock activity and Summary activity interchangeably. I do agree that largely they have the same function on the project, but objectives are different for adding a summary and hammock activity. So before going into detail a quick overview of the Summary Activity is as follows;

Summary Activity

Summary activity is simply a roll-up of activities defined under the Summary activity in a logical way using the Schedule Relationships.

Hammock Activity

A Hammock Activity aka hanging activity is made by grouping smaller subtasks that are not related to each other in one activity, that hangs between two dates i.e earliest start date and latest end date which portrays that it doesn’t have its own duration or size and it is dependent on the activities grouped under this. It can be done because the individual sequence is not important or logical in a hierarchal sense of WBS

The size of the hammock or the duration of the hammock is calculated by the start date of the earliest task, and the finish date of the last task.

The name hammock activity comes from the fact that individual activities hang between the overall dates, floating, just like a hammock.

Advantages of Hammock Activity

1. Help to see project activities on a summary level by ignoring the low-level activities and focusing on the overall results of these activities.
2. It Helps to minimize the total number of activities on the project to be displayed
3. It could prove to be a good reporting overview without getting overwhelmed by details.
4. hammocks can be used to carry overhead costs and resources for a section of the project, rather than trying to spreading them out across all of the associated activities.
5. The hammock’s duration is flexible and adjusts automatically as the underlying logic in the schedule changes
6. its connection points do not need to be in the same part of the schedule’s coding/summarisation structure

Further Readings

1. Project Schedule Management
2. Critical Path Method vs Critical Chain Method
3. Dangling Activities
4. LOE Activities
5. Summary & Hammock Activity
6. Hammock, Summary and LOE Activities