This chart explains why the stock market drop in May is not like the drop in December

There's a key measure of stock market volatility that shows why this month's sell-off has been different than the drop in December. The CBOE Volatility Index, or VIX, is sometimes known as "the fear gauge." The VIX helps measure Wall Street's expectations for large shifts in the stock market in the coming months by looking at prices on S&P 500 options. The VIX is near 18.50 on Friday – close to its 200 day moving average of 16.98.