This model for relative prioritising and sizing of requirements is a technique often used in an Agile/SCRUM environment, but I have found it to be invaluable in many different situations.
It is a way of quickly mapping out the relative priority and complexity or size of the items identified, so that decisions can be made about what to action. Rather than thinking about what the priority of each item should be, or estimating the work involved, the items are placed on a scale, based on how they sit in relation to other items on that scale. For example you don’t need to know the exact weights of an apple and a melon, to immediately realise that one is most likely to be about three times bigger and heavier.
As you can see in the image on the pdf, the horizontal axis represents the size or complexity of the task. The vertical axis represents priority. Ask the group to place each of the items identified during the brainstorming session on the matrix. Lots of discussion around why one thing is considered more or less complex, or higher priority than another, will help increase the shared understanding amongst the group. When all the items have been placed, decisions can more easily be made on which items will be actioned, by who and when.
The idea is that our brains are quicker at determining comparisons than worrying about calculating actual amounts.