Is there empirical data about a relationship between bitcoin price and difficulty? There have been claims that Bitcoin price and difficulty correspond. Is there empirical data to support or refute this?
If you look at the network hash rate, you'll notice that it fell off after the Bitcoin exchange rate dropped.

This leads to the conclusion that price drives difficulty. A drop in price caused a drop in difficulty. I have noticed this same correlation on another occasion; about two months ago.

Also, the reverse is probably true. When the price soared to $30+; the network hash rate accelerated greatly. The higher prices clearly attracted miners.