just tracks the inverse of price and has no predictive power. According to some, VIX should have predictive power as long as the prices computed by the Black-Scholes equation are valid assumptions about the volatility predicted for the future lead time (the remaining time to maturity). Robert J. Shiller has argued that it would be circular reasoning to consider VIX to be proof of Black-Scholes, because they both express the same implied volatility, and has found that calculating VIX retrospectively in 1929 did not predict the surpassing volatility of the Great Depression—suggesting that in the case of anomalous conditions, VIX cannot even weakly predict future severe events. An academic study from the University of Texas at Austin and Ohio State University examined potential methods of VIX manipulation. On February 12, 2018, a letter was sent to the Commodity Futures Trading Commission and Securities and Exchange Commission by a law firm representing an anonymous whistleblower alleging manipulation of the VIX. Volatility of volatility In 2012, the CBOE introduced the "VVIX index" (also referred to as "vol of vol"), a measure of the VIX's expected volatility. VVIX is calculated using the same methodology as VIX, except the inputs are market prices for VIX options instead of stock market options. The VIX can be thought of as the velocity of investor fear. The VVIX can be thought of as the acceleration of investor fear. See also Economic Policy Uncertainty Index Greed and fear Hindenburg Omen IVX Market trend Probability of default S&P/ASX 200 VIX SKEW Volfefe index References Further reading Black, Fischer and Myron Scholes. "The Pricing of Options and Corporate Liabilities." Journal of Political Economy (May/June 1973), pp. 637–659. "Amex Explores Volatility Options," International Financing Review, August 8, 1992. Black, Keith H. "Improving Hedge Fund Risk Exposures by Hedging Equity Market Volatility, or How the VIX Ate My Kurtosis." The Journal of Trading. (Spring 2006). Connors, Larry. "A Volatile Idea." Futures (July 1999): p. 36—37. Connors, Larry. "Timing Your S&P Trades with the VIX." Futures (June 2002): pp. 46–47. Copeland, Maggie. "Market Timing: Style and Size Rotation Using the VIX." Financial Analysts Journal, (Mar/Apr 1999); pp. 73–82. Daigler, Robert T., and Laura Rossi. "A Portfolio of Stocks and Volatility." The Journal of Investing. (Summer 2006). Fleming, Jeff, Barbara Ostdiek, and Robert E. Whaley, "Predicting Stock Market Volatility: A New Measure," The Journal of Futures Markets 15 (May 1995), pp. 265–302. Hulbert, Mark,