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Implied Volatility

Implied Volatility

Implied volatility (IV) is the single most important number in a systematic, premium-selling approach to options. It is the market's forward-looking, annualized estimate of how much an underlying is expected to move, backed out of live option prices — and because the strategy trades as a net seller of premium, knowing whether IV is high or low (and whether it is likely to rise or fall) drives nearly every entry decision. This section defines IV precisely, explains IV Rank versus IV Percentile, lays out the "high-IV, sell-premium" house rule, and connects mean reversion, vega, term structure, skew, and the VIX into one coherent framework.

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What Implied Volatility Is

Implied volatility is the volatility input that, when plugged into an option-pricing model (e.g., Black-Scholes), reproduces the option's current market price. Rather than being calculated from history, it is implied by what traders are willing to pay right now — making it forward-looking.

Three properties define how premium sellers use it:

The Expected Move

To translate annualized IV into a dollar range for a shorter horizon, traders use the expected move:

where DTE is days to expiration. This produces the ±1 standard deviation range (~68% probability) over that period.

Worked example. A $100 stock with 30% IV and 30 DTE has an expected move of `100 × 0.30 × √(30/365) ≈ $8.60`, i.e., a ~68% chance of finishing between $91.40 and $108.60 at that expiration.

The square-root-of-time term reflects that volatility does not scale linearly — random daily moves partially cancel rather than compound.

The "Rule of 16"

A fast shortcut: divide an annualized IV (or the VIX) by 16 to approximate the expected one-day percentage move. The 16 comes from √252 ≈ 15.87 (≈ the number of trading days in a year). So a VIX of 16 implies ~1% daily SPX moves; a VIX of 32 implies ~2%.

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IV Rank vs. IV Percentile

A raw IV number (say, 30%) is meaningless without context — 30% might be sky-high for one stock and rock-bottom for another. The fix is to normalize IV against the underlying's own past year using two distinct metrics.

IV Rank (IVR)

IV Rank measures where current IV sits within its 52-week high–low range:

Worked example. If XYZ's IV has ranged from 30 to 60 over the past year and currently trades at 45, its IV Rank is 50% — IV is exactly mid-range.

Because IVR is a range calculation, it can read above 100 or below 0 intraday if IV breaks its prior 52-week extreme before the high/low is updated.

IV Percentile (IVP)

IV Percentile measures the percentage of trading days over the past year on which IV closed below the current level:

An IVP of 80 means IV has been lower than it is today on 80% of the past year's days.

How They Differ

Why the distinction matters. A stock that spent most of the year at moderate IV but had one brief crisis spike can show a low IV Rank (because that spike inflated the range's top) while showing a high IV Percentile (because IV is still above most days). IVP "spots abnormalities better" and adjusts when volatility establishes a new normal; IVR is more intuitive for gauging position within a range.

Conflict/limitation to state plainly: the default and most-quoted metric across most platforms and educational content is IV Rank, and the bulk of the house rules are framed in IVR terms. However, dedicated data-science research has argued IV Percentile is the technically superior measure precisely because it is robust to one-off spikes. The two can disagree, and that disagreement is itself information — not a flaw.

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The House Rule: Sell Premium When IV Is High

The core thesis of the premium-selling approach: sell options premium when implied volatility is elevated. When IV is high, options are richly priced, so a seller collects more credit, gets a wider break-even cushion (a bigger expected move to be wrong within), and benefits if IV later falls.

The commonly taught threshold is IV Rank > 50 — i.e., IV in the upper half of its yearly range favors premium-selling strategies (short strangles/straddles, credit spreads, iron condors). When IVR is low, sellers either stand aside or shift toward defined-risk/long-premium or directional structures.

Important nuance: "> 50" is a widely repeated rule of thumb, not a hard mechanical law. Different sources cite different operating thresholds depending on context. Treat 50 as the canonical reference line, not a precise constant.

Why the edge exists: the volatility risk premium

Premium selling is profitable on average because implied volatility tends to overstate subsequent realized volatility. Options buyers are effectively buying insurance, and insurance must be priced above expected payouts to attract sellers — the gap is the volatility risk premium (VRP). Widely cited research finds that IV exceeds realized volatility roughly 80–85% of the time, which is the structural source of the seller's edge.

Honest caveat: the precise "~85%" figure circulates heavily in premium-selling education and corroborating sources, but it varies by underlying, period, and methodology. Treat it as a robust directional finding (IV usually > RV), not an exact constant.

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Mean Reversion, Expansion/Contraction, and Vega

Volatility mean-reverts

Unlike price, volatility is mean-reverting: extreme highs tend to fall back and extreme lows tend to drift up toward a long-run average. This is the engine behind the high-IV rule — selling when IV is high gives you a tailwind if IV reverts down toward its mean.

Vega: the bridge from IV to P&L

Vega is the Greek that quantifies IV exposure: the change in an option's price for a 1% change in implied volatility in the expiration being traded.

A short premium seller is structurally short vega: they win if IV contracts (and lose if IV expands against them). Vega affects only an option's extrinsic value, which is why options get more expensive in uncertain, high-IV environments.

Putting it together: a seller wants to be short vega when IV Rank is high, collecting inflated premium and profiting as IV mean-reverts down (vol crush) while theta decays the option.

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Term Structure and Volatility Skew

IV is not a single number across an underlying — it varies by expiration (term structure) and by strike (skew). Together these form the volatility surface.

Term structure: contango vs. backwardation

Plotting at-the-money IV across expirations reveals the term structure:

In VIX futures specifically, contango dominates the large majority of trading days, with backwardation appearing in a minority of stressed sessions — a practical signal that short-dated premium is unusually rich relative to long-dated.

Volatility skew (put skew)

Skew describes how IV changes across strikes at a given expiration. In equity-index and most stock options, out-of-the-money puts carry higher IV than equidistant out-of-the-money calls — the "put skew" (a.k.a. reverse/negative skew).

Why puts are bid up:

Practical consequence: a put sold at the same delta as a call typically collects more premium because of skew — a structural reason index/equity sellers often lean to the put side.

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VIX: The Benchmark

The VIX ("fear gauge") is the market's expectation of 30-day volatility of the S&P 500 (SPX), derived from a strip of SPX option prices. It is, in effect, the IV of the broad market.

Caveat: VIX measures index volatility. Single-name IV Rank can diverge sharply from VIX, so per-underlying metrics — not VIX alone — should drive single-name trade selection.

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Key Takeaways

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Common Misconceptions

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Review Questions

1. Write the expected-move formula and compute the ~68% range for a $200 stock with 25% IV at 45 DTE.

2. A stock's IV ranged from 20 to 80 this year and now sits at 35. What is its IV Rank? In one sentence, why might its IV Percentile tell a different story?

3. State the house rule for when to favor premium selling, and explain the volatility-risk-premium reason it works.

4. A short strangle is held into an unexpected market shock and IV expands sharply. Using vega, explain the immediate effect on the position before any move in the underlying.

5. Define contango and backwardation in the volatility term structure, and say which one typically signals market stress.

6. The VIX is 24. Using the Rule of 16, what is the approximate expected one-day move in the S&P 500, and why is 16 the divisor?

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Sources

_Evidence-labeled per the Project Charter. Education only, not financial advice._