I mean they were pulled out of the ether by the authors. Their personal whims only.
@Bigun @skeeter - any measurement could be considered arbitrary. Why pick to measure length and width, for instance?
We measure length and width because it tells us something useful and predictable. If we know what the length and width are, we can predict how much stuff we can cram into that space.
Likewise, using these 5 measurements, we can predict a lot of things - behaviors, characteristics....and even politics. But, it has to be repeatable and accurately predict these things.
Make sense?
Sorry,
@endicom, I should have pinged you too.