A couple of week's ago, I was writing about CPRE's report highlighting a renewed 'threat' to the greenbelt.
The report identifies projects "amounting to the development of a new
town greater than the size of Slough over the next twenty years". At the time, I commented on the fact that this seemed an odd unit of measurement.
One might argue that it's a simple way of communicating the threat. However, Daniel Kahneman's Thinking, Fast and Slow provides an alternative explanation - that CEPR are suffering from 'denominator neglect' (or attempting to make use of it). Denominator neglect in this cases means that your attention is drawn to the new development, but you ignore all of the greenbelt land left untouched. Denominator neglect is the effect that causes people to over-weight low probability events.
A number of other biases are at work when it comes to assessing statements about the greenbelt. I'm not sure that I completely understand all of these, but I think I've understood enough to know that I will henceforth discuss proposals for the greenbelt as follows: 'Over the next twenty years we plan to build 1 million more homes, helping address housing affordability for our poorest families; our plans safeguard the countryside ensuring that 99.5% of our currently undeveloped land will remain undeveloped.'
When I get a moment, I'll do the exact maths in terms of 1 million houses and the amount of land left undeveloped - but I'm hoping the re-framing might help regardless.