A human barometer is a great way to capture a broad range of opinions and to allow small changes of view to be measured during a discussion. The barometer is a patricularly good technique to use if an issue is not clearly black and white.
Set up the room with one side labelled ‘for’ and the opposite side labelled ‘against’, then ask your students to stand between the markers at the point where their opinion lies.
This can be repeated after the discussion. You could take a photo of the barometer using a digital camera to compare the differences in individual’s opinions before and after the discussion.
If moving people around the room is not practical, create the barometer on a wall or a whiteboard and use sticky notes or coloured card to mark where students’ opinions lie.