SIE Ready⚠️ Investment Risks · Lesson 1 of 5 · 5 min
The Risk You Can Diversify and the Risk You Can't
🌊 A rising tide lifts all boats, and a falling one sinks most of them. Owning 500 stocks protects you from one bad company, not from a bad market.
💡 Key idea
Diversification shrinks unsystematic (company) risk, never systematic (market) risk. Beta measures exposure to market moves.
🧠 Why it matters
SYSTEMATIC (market) risk hits the whole market or economy: recessions, broad sell-offs, interest-rate and inflation shocks. Diversifying across many stocks cannot remove it. UNSYSTEMATIC risk belongs to one company or industry: a failed product, a lawsuit, poor management (business risk), or a rule change aimed at one sector. Spreading money across many unrelated holdings can greatly reduce it. BETA measures a stock's historical volatility relative to the market, so it gauges systematic risk. Beta 1.0 = moves with the market; 1.5 = tends to move about 50% more than the market in either direction; 0.5 = about half as much. A beta above 1 is not "better" or "worse"; it means larger swings both ways.
📈 In the real world
📉 Jordan owns only Brightline Airlines, which grounds its fleet after a safety probe; her stock drops 40% while the market is flat. That is unsystematic risk. A year later a recession drags her diversified index fund down 20%. That is systematic risk, and no number of extra holdings would have prevented it.
📌 Takeaways
- Systematic = market-wide and not diversifiable
- Unsystematic = one company or industry, reduced by diversification
- Beta > 1 swings more than the market; beta < 1 swings less
📖 Terms in this lesson
Systematic risk: Risk that affects the whole market and cannot be removed by diversification.
Unsystematic risk: Risk specific to one company or industry, which diversification can reduce.
Beta: A measure of a stock's volatility compared with the market; a beta of 1.0 moves in line with the market.
SYSTEMATIC = THE WHOLE SYSTEM: Systematic risk hits the whole system, so diversification cannot remove it; unsystematic risk is one company's, so diversification can.
✅ Test yourself
Which risk can be substantially reduced by owning many stocks across different industries?
- Interest rate risk
- Market risk
- Business risk
- Inflation risk
Answer: C · Business risk
Business risk is company-specific (unsystematic), so diversification reduces it. Market, inflation and interest rate risk affect the whole market and are systematic.
A stock has a beta of 1.4. If the overall market falls 10%, the stock would be expected to:
- Fall about 4%
- Rise about 14%
- Fall about 14%
- Fall about 10%
Answer: C · Fall about 14%
Beta 1.4 means the stock has tended to move about 1.4 times the market, so 1.4 × 10% ≈ 14% down. It moves in the same direction as the market, not opposite, and 10% would be a beta of 1.0.
Beta is a measure of which type of risk?
- Credit (default) risk
- Unsystematic (company) risk
- Liquidity risk
- Systematic (market) risk
Answer: D · Systematic (market) risk
Beta compares a security's movements with the market's, so it measures exposure to systematic risk. It says nothing about how easy a security is to sell, an issuer's default odds or company-specific events.
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