Although this test successfully detects single isolated bubbles, Phillips, Shi, and Yu [24] acknowledge it may suffer from reduced discriminatory power when applied to time series with multiple occurrences of bubbles. To overcome this weakness, a further enhancement was proposed, as a new method, called a generalized supremum ADF (GSADF) test. This test allows both the start and end points of data subsets to vary, which in turn enables the identification of multiple bubble regimes in one observed time series. The GSADF test is defined by
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In 2016, this became apparent with the DAO hack. One way to fix the problem was to implement what’s known as a “hard fork,” which would essentially update the Ethereum-based software to fix the technical gaffe that caused the hack to begin with. But DAO users had to agree to this change, and there were dissenters. Though the hard fork was approved, it created two active blockchains with two different sets of rules. Ultimately, this hack–coupled with the inability to deal with it–caused the DAO to end in 2016.
But while it’s based on sound, democratic principles, cryptocurrency remains a technological and practical work in progress. For the foreseeable future, nation-states’ near-monopoly on currency production and monetary policy appears secure.