You also need to assemble this group from a random selection of people from all across the country. The context of the analogy attempts to get the reader to visualize a room of 400 people which is easiest to do by drawing on personal experience such as a school assembly. But the stats listed will not apply to a group of 400 people from the same school zone, the same age. But visualizing a group of people you have an empathetic connection to is effective because it makes you wonder "what if a large amount of my friends of these disadvantaged people?" So it makes the message more effective, but it is utilizing emotional manipulation to do so.