Today the VIX went up 25% in one day! Anyway, I am thinking what determines the value of the VIX? I realize that mathematically it is the calculation of the implied volatility from the Black-Scholes equation, based on the price of SPY options, interest rate and dividends. But why... ...is it that when the S&P 500 goes down the VIX also usually goes up, but with no basic pattern. Sometimes it can go up 5% when the S&P 500 goes up 2%, but other times it can go up 10% or more. So why??????