The researchers defined a bubble as a sharp price run-up over a two-year followed by at least a 40% drop over the subsequent two years. When the price run-up is 100% or more, they found the probability of a crash becomes 50%. When focusing on price run-ups of at least 150%, that probability becomes 80%. As price run-ups become even bigger, a crash becomes “nearly certain.”
https://www.marketwatch.com/amp/story/guid/6D6CF0C8-D3AF-11E7-8E50-41CEEA71C61F
