difference in y / difference in x.
Why nothing ever goes up forever? Because x cannot be 0! No matter what bullshit the media sprouts about housing going up forever, it's not sustainable. Sure, it can go on for a long time. Years even. But nothing ever rises forever, simply because of that mathematical fact.
Put simply, plot a graph of X vs Y. Imagine Y to be the price of housing and X to be demand. For Y to keep rising, X has to keep rising. Now, what is X? People. People buying. And there's a finite number of people, yes? Ah ha... now if everyone in Australia buys a house, what do you think will happen then? Is there a true housing glut? Oh really, how many people are speculating now buying several houses?
Bullshit is what I call it.