During earnings season it is common to see large single-stock moves every night and an index that barely changes. One company jumps, another drops, and the S&P 500 closes flat. Index volatility is low. Stock volatility is high. Both are accurate.
An index is an average
An index moves by the weighted sum of its members. When members move in different directions, their moves cancel. When they move together, nothing cancels and the index moves as much as they do.
So index volatility depends on two things: how volatile the stocks are, and how closely they move together. The second is correlation. Low correlation can hold index volatility down even when every stock in it is active.
Why earnings season does this
Earnings news is company-specific. One firm’s good quarter says little about another firm in a different industry. Reactions are large and mostly unrelated to each other. That is a period of high stock volatility and low correlation, and the index averages it away. Earnings season covers the calendar side.
The exception is a company large enough to move the index alone, the case in NVDA earnings night.
What options imply
The same relationship holds in option prices. Index options and single-stock options each carry implied volatility. Compare them and you can back out the correlation the market is pricing among the members.
A low index figure beside high single-stock figures means the market is pricing stocks to keep moving independently. Trades built on the gap between the two are called dispersion trades.
How the calm ends
A macro shock moves everything at once. A rate scare, a credit event or a geopolitical headline does not care which industry a company is in. Stocks that were offsetting each other start falling together. Correlation rises, and index volatility rises with it even if no single stock is more volatile than before.
This is why a low VIX reading supported by low correlation can change quickly. The stocks were never calm. The offsetting was doing the work, a point made from another angle in a low VIX does not mean low risk. Correlation risk takes the portfolio view.
What to watch
Breadth is the simple check. On a day when most stocks fall together, correlation is rising. On a day when winners and losers are evenly split and the index is flat, it is low. A change from the second kind of day to the first is the thing to notice.
Where NoVo shows it
NoVo’s free volatility page ranks VIX, VXN and RVX against their own history, so a calm index reading can be seen for where it sits. The Trader dashboard shows dealer positioning and the expected move on SPY, QQQ and IWM, with written reads from Dr. NoVo, the Financial Markets Super Intelligence.