The index you watch on the news and the index you can actually trade are rarely the same instrument. Learn the gap and you stop being surprised by "tracking error".
2.1How is a stock index actually built?
An index takes a list of securities and combines their prices into one figure using a weighting method. The method decides which members actually move the number — and that has real trading consequences.
Price-weighted (the Dow)
The oldest method. You add up the share prices and divide by a divisor. A $400 stock therefore swings the index four times as hard as a $100 stock, regardless of how big the companies are. It's a quirk of history more than logic.
Three stocks at $400, $150, $50; divisor = 0.152 (divisors are tiny because of decades of splits):
Now the $400 stock rises 10% (+$40): new sum 640 ÷ 0.152 = 4,210.5, a +6.7% index move. The same 10% rise in the $50 stock (+$5) gives 605 ÷ 0.152 = 3,980.3, just +0.8%. Same percentage move, eight times the index impact — purely because of price, not size.
Capitalisation-weighted (the S&P 500, Nasdaq-100)
The modern standard. Each member is weighted by its market capitalisation (price × shares), usually float-adjusted so only freely-tradable shares count. Big companies dominate. This is why a handful of mega-cap tech names can drag the whole S&P 500 up or down while the median stock barely moves.
A company worth $3.0tn in an index whose members total $50tn:
Stack the top 7 names at ~30% combined weight and you see why "the index" and "the average stock" can tell completely different stories on the same day.
Equal-weighted
Every member gets the same slice (e.g. the S&P 500 Equal Weight, ticker RSP). It removes mega-cap dominance and tilts toward smaller members — useful for gauging market breadth. When the cap-weighted S&P rises but the equal-weight lags badly, the rally is narrow and fragile.
2.2The major equity indices — and how to trade each
For every index there are typically three routes: futures (leveraged, nearly 24-hour, capital-efficient), ETFs (simple, fully-funded, in any brokerage account), and options (on the index, the future, or the ETF). Here's the US line-up.
| Index | Future (full / micro) | Main ETF(s) | Character |
|---|---|---|---|
| S&P 500 | ES / MES ($50 / $5 × idx) | SPY, VOO, IVV | The benchmark. Broad large-cap, the deepest pool of liquidity on Earth. |
| Nasdaq-100 | NQ / MNQ ($20 / $2 × idx) | QQQ | Tech-heavy, higher beta, more volatile than the S&P. |
| Dow 30 | YM / MYM ($5 / $0.50 × idx) | DIA | Price-weighted, old-economy tilt, narrow (30 names). |
| Russell 2000 | RTY / M2K ($50 / $5 × idx) | IWM | Small-caps. Domestic, rate-sensitive, the "risk appetite" tell. |
ES (and its micro MES) is the most actively traded equity index future in the world, regularly turning over hundreds of billions of dollars of notional a day, and SPY is one of the highest-volume securities of any kind. That depth is why slippage on these is near-zero and why they're the default vehicle for expressing a quick macro view. Verify current volumes with CME / the exchange.
| Catalyst | Simple reaction | The non-obvious second-order |
|---|---|---|
| Fed cuts rates | Stocks up (cheaper money, higher valuations) | If the cut is read as panic about growth, stocks can fall — "why are they cutting? what do they see?" |
| Strong jobs report | Good economy → stocks up | In an inflation regime, strong jobs = Fed stays tight → stocks down. "Good news is bad news." |
| Mega-cap earnings beat | Index up (heavy weights) | If the beat is "priced in", the stock can sell the news and drag the cap-weighted index even as breadth is fine. |
| Yields spike | Stocks down (discount rate up) | Long-duration/tech hit hardest; value and banks can rise — it's a rotation, not just a sell-off. |
International indices — the same template
Outside the US you'll meet the Euro Stoxx 50 (FESX, the eurozone blue-chip benchmark), DAX (FDAX, Germany — a total-return index, so dividends are reinvested into the level, which trips people up on comparisons), FTSE 100 (Z, UK large-caps, heavy in energy/miners/banks and very sensitive to the pound), Nikkei 225 (price-weighted, like the Dow, and yen-sensitive) and Hang Seng (Hong Kong, a liquid proxy for China risk). Each trades via local futures and via US-listed ETFs (e.g. EWG Germany, EWU UK, EWJ Japan, FXI/MCHI China).
Index futures carry the cost-of-carry from Part 1.4: future ≈ spot + financing − dividends. The visible gap between the future and the cash index is fair value, not free money. Before the US cash open, traders watch ES against fair value to gauge where stocks will open. Also note the quarterly roll (Mar/Jun/Sep/Dec): liquidity migrates to the next contract during the week before expiry, and holding the expiring month too long means trading a thinning book.
2.3The VIX, in depth — what the "fear index" really measures
The VIX is the market's expectation of how much the S&P 500 will move over the next 30 days, expressed as an annualised percentage. It is not a price you can buy directly — it's calculated from the prices of a whole strip of S&P 500 (SPX) options. When investors rush to buy protection, option prices rise, and the VIX rises with them. That's why it spikes in crashes and drifts low in calm markets.
Where the number comes from (conceptually)
You don't need the full integral to trade it, but the intuition matters. The VIX blends the prices of many out-of-the-money SPX puts and calls across two expiries bracketing 30 days, weights them, and converts the result into an annualised volatility. More expensive options (especially puts) → higher implied variance → higher VIX. Roughly:
There are ~252 trading days a year, and √252 ≈ 15.9, so dividing by 16 is the desk shortcut. With VIX = 16:
So a VIX of 16 says "expect roughly ±1% days". A VIX of 32 implies ±2% days; a VIX of 80 (March 2020) implied ±5% daily swings. This one-line conversion is the single most useful thing to memorise about the VIX.
The VIX term structure
Just like commodities, volatility has a curve. In calm markets the VIX futures curve is in contango (further-out vol priced higher than spot) because uncertainty grows with time. In a crisis it flips to backwardation (spot fear > future fear). That shape drives the products below.
How do you actually trade the VIX?
- VIX futures (
VX): the only direct way to trade expected volatility. They settle to the VIX at expiry but trade at their own curve-implied levels before then. - VIX options: options on the VIX future — popular for cheap, convex tail hedges.
- ETPs:
VXX/VIXY(long short-dated vol),UVXY(leveraged long),SVXY(short vol). These hold a rolling position inVXfutures — and that's where the trouble starts.
Long-vol ETPs roll daily from a cheaper near future into a pricier next future (contango = negative roll yield, Part 1.4). Say front VX = 18, next = 20:
Compounded over a calm quarter, the curve alone can erode the product by double digits % with the VIX itself unchanged.
This is structural, not a bug. VXX has reverse-split many times because it grinds toward zero in quiet regimes. Long-vol ETPs are short-term hedges or tactical trades, never buy-and-hold. The mirror image: SVXY (short vol) harvests that contango in calm markets — and then gets annihilated in a spike (see Feb 2018's "Volmageddon", when short-vol products lost most of their value in a day).
| Catalyst | Simple reaction | The non-obvious second-order |
|---|---|---|
| S&P sells off hard | VIX spikes (demand for puts) | The VIX–S&P relationship is asymmetric: vol rises faster on the way down than it falls on the way up ("up the stairs, down the elevator"). |
| Quiet, grinding rally | VIX drifts lower | A very low VIX can itself be a risk signal — complacency and crowded short-vol positioning set up violent unwinds. |
| Known event ahead (Fed, CPI, election) | VIX elevated into it | "Vol crush" — the VIX often falls the moment the event passes, even on a big move, as uncertainty resolves. Buying options into events can lose despite being right on direction. |
2.4The wider volatility & cross-asset family
The VIX has cousins across asset classes. Knowing them turns "stocks are scared" into a far richer cross-asset read.
MOVE — the VIX of the bond market
The MOVE index (Merrill Lynch Option Volatility Estimate) measures implied volatility of US Treasury options — it's the bond market's fear gauge. Rates desks watch it the way equity desks watch the VIX. A rising MOVE means the rates market expects big yield swings, which tightens financial conditions, widens credit spreads and tends to pressure risk assets even if the VIX is calm. Because so much of the financial system is priced off Treasuries, the MOVE is arguably the more important gauge for spotting systemic stress. You generally can't trade the MOVE directly; you express the view through Treasury options, swaptions, or rate-vol products.
The rest of the complex
| Gauge | Measures | Tradable? |
|---|---|---|
| VVIX | The volatility of the VIX — "vol of vol". High VVIX = the vol market itself is jumpy. | Indirectly, via VIX options. |
| SKEW | The relative cost of far out-of-the-money SPX puts — i.e. how much the market is paying up for crash protection (tail risk). | Indirectly, via put spreads. |
| OVX | Implied volatility of crude oil (the "oil VIX", built from USO options). | Via oil options. |
| GVZ | Implied volatility of gold (GLD options). | Via gold options. |
| DXY | The US dollar against a basket (mostly EUR, plus JPY, GBP, CAD, SEK, CHF). The single most important cross-asset variable. | Yes — DX futures, UUP ETF, FX. |
| Catalyst | Simple reaction | The non-obvious second-order |
|---|---|---|
| US rates rise vs the rest of the world | Dollar up (capital chases yield) | It's relative rates that matter — if Europe hikes faster, the dollar can fall even as US rates rise. |
| Global risk-off / crisis | Dollar up (safe-haven, the world's funding currency) | A strong dollar then tightens conditions for everyone with dollar debt — emerging markets and commodities get squeezed, feeding the risk-off loop. |
| Strong dollar | — | Commodities (priced in USD) tend to fall; this is why DXY belongs on every commodity trader's screen. |
2.5Reading the gauges together
No single gauge tells the whole story. The skill is in the combinations:
- VIX up + MOVE up + credit spreads widening + DXY up: genuine systemic stress. Everything is repricing risk at once. This is when correlations go to 1 and "diversified" books still lose.
- VIX up but MOVE calm: an equity-specific wobble, not a macro event. Often a buyable dip rather than a regime change.
- VIX very low + SKEW very high: the surface looks calm, but smart money is quietly paying up for crash insurance. A classic late-cycle tell.
- Equal-weight lagging cap-weight badly: a narrow rally led by a few mega-caps — strong index, weak market.
None of these forecast the future on their own — they price the market's current expectation. Their value is in divergences and regime shifts: when the bond market (MOVE) is screaming while equities (VIX) are calm, trust the bond market. Rates lead, equities follow, and credit is the canary. A veteran's dashboard is rarely about the level of any one gauge; it's about whether they agree.