Remember, the goal of the Jam is to produce something—even if it the design never becomes a realworld solution. The Jam is about doing, not about talking.
Now, some people at your Jam—especially designers, programmers, project managers—will have experience of going through a development process, starting with a challenge or brief and ending up with an actionable result at the deadline. Other people will not have this experience. So, depending on your Jammers, you might decide to help them keep moving forward by offering an activity structure, like the design process as shown here in a great version by Make.
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This video shows how that structure came to life in the Gothenburg group at the Global Service Jam 2011 (GSJ11).
If you choose this more structured Jamming, you might decide to divide the Jam into phases, and encourage the Jammers to work through the steps according to your time plan. A little bit of structure seems to help most Jams… there are a couple of example time plans in this handbook, and there are even more at the Organisers’ Basecamp.
Of course, you might just provide an open space and let your Jammers go for it. It’s your Jam!