On Monday at 9:50, Sam's scanner lit up with three chip stocks at once: GHIJ, NOPR and STVW, all up on the same industry news. Each one passed every check from last week. Clean stop, correct size, a 1:2 ratio (Lesson 56: Win Rate vs Risk/Reward).

Three good setups felt like three chances. He took all three, $60 of risk each.

At 10:12, a headline hit the whole industry. All three dropped in the same two minutes, and all three stops triggered within 40 seconds of each other.

In 30 seconds

  • Correlation measures how closely two prices move together, from −1 (opposite) through 0 (unrelated) to +1 (in lockstep).
  • Correlation risk is the danger that several positions move together because they share a driver, such as the same SectorA broad grouping of companies that operate in the same segment of the economy — such as Technology, Healthcare, Energy, or Financials. Stocks within a sector tend to move together in response to shared economic, regulatory, and sentiment faRead definition → or news, so separate trades behave like one bigger trade.
  • Correlations between stocks tend to rise in sell-offs, which is exactly when you need them to be low.
  • A simple rule: count positions in the same sector or story as one trade, and cap their combined risk at your normal 1R, or at whatever is left of your daily max loss.

In this lesson you'll learn what correlation means, how holding similar stocks multiplies your risk, why correlations climb when markets fall, and how to size several positions so one headline can't break your daily limit. It builds on Lesson 41: Sector and Market Context.

What is correlation in trading?

Correlation is a measure, from −1 to +1, of how closely two prices tend to move together: +1 means they move in lockstep, 0 means there's no consistent relationship, and −1 means they move in opposite directions.

Think of umbrellas and sunscreen at a beach shop. Rain sells one and hurts the other, so they're negatively correlated. Umbrellas and raincoats both sell on rainy days, so they're positively correlated. Owning a shop full of raincoats and umbrellas isn't diversified. It's one big bet on rain.

Stocks in the same industry, or reacting to the same news, are the raincoats and umbrellas of your watchlist. Investor.gov describes diversification as spreading your money across investments that don't all move the same way, and three stocks that react to one headline fail that test.

Illustrative: diversification needs different drivers, and the ticker symbols don't count.

One detail trips people up. Correlation measures direction, and it says nothing about size. Two stocks with a correlation of 0.9 usually move the same way at the same time, but one can still move 8% while the other moves 2%.

How does correlation multiply your risk?

Correlation multiplies your risk because positions that share a driver tend to hit their stops together, so three trades sized at 1% each can lose about 3% in one move.

Each of Sam's positions was sized correctly on its own. Together, they were one $180 bet on chip stocks, three times his normal risk and above his $120 daily max loss (Lesson 54).

Illustrative: three 1% positions with one shared driver behave like one 3% bet.

The DayTradingToolkit simulation below shows what a shared driver changes. Each batch is three trades with Sam's plan from Lesson 56: a 40% chance of winning 2R, otherwise a 1R loss, with costs of about $3.60 a trade. The only thing that varies is how much of each stock's move comes from a common driver.

Setup
3 trades × $60, unrelated stocks
Chance of losing $120 or more in the batch
22%
Average batch
+$25
Worst case
−$191
Setup
3 trades × $60, moderate shared driver
Chance of losing $120 or more in the batch
38%
Average batch
+$25
Worst case
−$191
Setup
3 trades × $60, strong shared driver (same sector, same news)
Chance of losing $120 or more in the batch
49%
Average batch
+$25
Worst case
−$191
Setup
3 trades × $20, strong shared driver, one group budget
Chance of losing $120 or more in the batch
0%
Average batch
+$8
Worst case
−$64
Setup
1 trade × $60, best name only
Chance of losing $120 or more in the batch
0%
Average batch
+$9
Worst case
−$64

DayTradingToolkit simulation: 1,000,000 hypothetical batches per row, 40% Win RateWin rate is the percentage of your trades that result in a profit — calculated as (number of winning trades ÷ total trades) × 100 — and while commonly cited as a measure of trading success, it means very little without also knowing the averRead definition →, 1:2, fixed seed. Shared-driver levels correspond to latent correlations of 0, 0.6 and 0.9. Hypothetical, not real trader data.

Read the first three rows. The average barely moves, at about +$25 a batch whether the stocks are related or not. What changes is how often the worst case shows up: one batch in five with unrelated stocks, and one batch in two with a strong shared driver. Your average stays the same while your daily limit gets broken twice as often.

The bottom two rows are the honest part. Splitting the risk fixes the tail and also shrinks the average to about a third, because you're risking a third as much. You're paying a little expected profit for staying inside your daily max loss. For an account that can't afford to break its limit, that's a fair price.

Why do correlations jump when markets fall?

Correlations between stocks tend to rise in sell-offs, so positions that seemed unrelated on a calm day often fall together on a bad one.

This is one of the better-documented patterns in finance. Longin and Solnik found that correlation between international stock markets "increases in bear markets, but not in bull markets." Ang and Chen found that US stocks are much more correlated with the market during downside moves than upside ones.

For a day trader, that means a market-wide drop can turn a tech stock, a retailer and a bank into one position for an hour. That's why Lesson 41 had you check SPY before every trade.

What does a one-headline sector drop look like in real life?

It looks like a whole group of stocks falling at once, whatever each company does. On Monday, January 27, 2025, investors sold chip and AI stocks after the emergence of DeepSeek, a Chinese startup whose low-cost AI model raised fears for AI leaders such as Nvidia.

According to a wire report published that day, these were the moves:

Stock or index
Nvidia
Move on the day
down just under 17% (a record one-day market value loss of $593 billion for any company on Wall Street)
Stock or index
Broadcom
Move on the day
down 17.4%
Stock or index
Marvell Technology
Move on the day
down 19.1%
Stock or index
ASML (Europe)
Move on the day
down 7%
Stock or index
Philadelphia Semiconductor Index
Move on the day
down 9.2%, its biggest percentage drop since March 2020
Stock or index
Nasdaq
Move on the day
down 3.1%

Source: wire report of January 27, 2025, as published by The Daily Herald. Percentages are for that trading day only.

Notice two things. Companies of very different sizes and businesses moved on one story, so a trader holding three of them had one bet. And the drops weren't equal: Microsoft fell 2.1% and Alphabet 4.2% in the same report. That's the direction-versus-size point from earlier. Related stocks fall together, in different amounts.

How do you manage correlation risk?

You manage correlation risk by grouping positions that share a driver and treating each group as a single trade with a single risk budget.

  1. Group by driver. Same sector, same news story, same theme or heavy reliance on the same index. If one headline could hit them all, they're one group.
  2. One group, one risk budget. Split your normal 1R across the group. Two chip stocks at $30 each instead of $60 each.
  3. Cap your total open risk. The combined risk of everything you hold should never exceed what's left of your daily max loss.
  4. Prefer the strongest one. If three names move on the same news, pick the one with the best setup and LiquidityLiquidity in trading refers to how easily you can enter and exit a position at a fair price — a liquid stock has high volume and a tight bid-ask spread, allowing large orders to be filled quickly without moving the price, while an illiquid Read definition → instead of all three.
  5. Watch the driver. Keep the sector ETF or SPY on screen. When the driver turns, all your related positions are at risk together.

A sector ETF (an exchange-traded fund that holds a basket of stocks in one industry) is a quick way to see the group's move. Investor.gov's bulletin on ETFs explains how they work.

Not every pair of stocks in a sector needs the full treatment. Use the question that matters: could one headline stop both of these trades? If yes, share the budget. If the charts and catalysts differ (one gapped on EarningsAn earnings report is the quarterly financial disclosure a public company files with the SEC, revealing revenue, earnings per share (EPS), profit margins, and forward guidance — it's the most significant regular catalyst for individual stocRead definition →, the other is drifting with the market), judge each trade on its own, and keep the total open risk cap either way.

Sam's chip-stock morning

Here's what happened to Sam's three positions. Every ticker and figure is hypothetical, in his paper account.

Stock
GHIJ
Shares
100
Entry
$19.00
Stop
$18.40
Risk
$60
Exit
$18.39
Loss with fees
$61.04
Stock
NOPR
Shares
80
Entry
$24.00
Stop
$23.25
Risk
$60
Exit
$23.24
Loss with fees
$60.84
Stock
STVW
Shares
150
Entry
$12.00
Stop
$11.60
Risk
$60
Exit
$11.59
Loss with fees
$61.54
Stock
Total
Shares
Entry
Stop
Risk
$180
Exit
Loss with fees
$183.42

How the three trades should have been sized (hypothetical)

Normal risk for one trade$60
divided by Positions in the same group3
equals Risk per position$20, so 33 GHIJ, 26 NOPR, 50 STVW
Try this in the calculator →

With those smaller sizes, the same 10:12 headline would have cost about $60 across all three trades, half his daily max, instead of $183.42.

The loss of $183.42 went past his $120 daily max in a single move. No rule was broken trade by trade. The rule that was missing sat one level up.

Sam's card got a new line under "Per day": "Same story = same trade. Total open risk never more than what's left today."

Mistake and fix

Calling similar stocks diversification

You're doing it if
You hold three or four stocks from the same sector or news story and think the risk is spread out because the tickers are different.
The fix
Group positions by what drives them. Give each group one risk budget, and pick the best setup in a group instead of taking every name.

Your turn

Group your watchlist (10 minutes)

0 of 6 done

Check your understanding

Check your understanding

You hold three bank stocks, each with $50 of risk. What's your realistic risk if the whole sector drops on one headline?

Check your understanding

What does research show about correlations during market sell-offs?

Check your understanding

Your daily max loss is $120 and you've already lost $60 today. What's the most total open risk you should hold?

Key takeaways

  • Different tickers aren't diversification if one headline moves them all.
  • A shared driver leaves your average result alone and makes the worst case about twice as likely.
  • Group positions by their driver and give each group one risk budget.
  • Correlations rise when markets fall, so a market drop can join unrelated positions.
  • Keep total open risk within what's left of your daily max loss, and when several names move on the same story, trade the best one.

Frequently asked questions

How do I check if two stocks are correlated?
The quick way is to put both charts side by side on the same timeframe and see whether they rise and fall together. For a number, TradingView's Correlation Coefficient indicator compares your chart with a second symbol, or you can export recent 5-minute closes into a spreadsheet, turn them into percentage changes and use the CORREL function. Above about 0.7 is a strong positive relationship. Remember that it measures direction and not size.
Is buying a stock and its sector ETF a hedge?
No, if you buy both, that doubles the same exposure. A hedge means taking an opposite position, such as owning a stock and shorting its sector ETF, which is an advanced technique with its own risks and costs.
What if the setups look different, do I still have to split my size?
Not always. AskThe ask is the lowest price a seller is currently willing to accept. When you buy with a market order, you pay the ask.Read definition → whether one headline could stop both trades. If the charts and catalysts differ, such as one stock gapping on its own earnings and another drifting with the market, treat them as separate trades. If they look nearly identical, take the best one at full size or split the risk across them. Either way, your total open risk still has to fit inside what's left of your daily max loss.
How many positions should I hold at once?
There's no fixed number. The limit is your risk, not the count. With a $120 daily max loss and $60 per trade, two full-risk trades open at once would already use Sam's whole limit. Smaller sizes let you hold more positions, but each group still shares one budget.
Are long and short positions in the same sector safe from correlation?
They offset each other partly, but not reliably. Two stocks in one sector can still split on company news, so a long in one and a short in another can both lose. Treat it as two trades unless you understand exactly what you're hedging.
Does correlation stay the same over time?
No. It shifts with news, market conditions and time of day, and it tends to rise during sell-offs. A pair that looks independent this month can move together next month.

Up next: where exactly should a stop go?

Looking at Monday's table that evening, Sam noticed something else. His stops were 60, 75 and 40 cents away: round numbers he'd picked to make the shares come out neatly. None of them sat at a level on the chart. Were they in the right places at all? Lesson 58: How to Place a Stop Loss Correctly shows how to choose one.

Disclaimer

This lesson is for educational purposes only and is not financial advice. Correlations change over time and tend to rise in sell-offs, so grouping rules reduce but don't remove the risk of several positions losing together. Hedging with short positions or ETFs carries its own risks. The simulation is hypothetical and doesn't predict results. All Sam examples and figures are hypothetical and in a paper account. Trading involves the risk of loss, and nothing here guarantees any result. For our complete disclaimer, visit https://daytradingtoolkit.com/disclaimer.

Article Sources

This lesson relies on peer-reviewed research, SEC investor guidance, a news report on a real sector sell-off and an original DayTradingToolkit simulation.