Just The Markets

Game · Investing · Beginner

Build It, Then Break It: A Portfolio Stress Test Game

Build a hypothetical portfolio, then put it through a portfolio stress test game of rate hikes, sector selloffs and a scare. The value moves with your allocation; the score moves only with your decisions.

AI-assisted, reviewed by the Just The Markets human editor: Lovely Oryza → Scored on reasoning Published

What it trains

Whether you can hold a portfolio through made-up shocks by rebalancing to a rule, without panic selling or chasing the latest winner.

Split a hypothetical $100,000 across the sleeves before you see a single headline. That is the build. Then the shocks arrive, one per round, each with a percentage move for every sleeve, and the value in the corner changes with whatever you chose. The moves are made up for the game and describe no real market.

What you are practicing

You are practicing holding. A shock does its worst damage through the trade made afterward: the sale at the low, the purchase at the high, the whole portfolio rebuilt around last month. The rule you play with is a drift band. Rebalance any sleeve that moves 5 percentage points or more from its target, and leave the rest alone. The band is wide on purpose. Small drifts cost more to fix than to live with, since every trade pays a spread and, in a taxable account, may bring a tax bill.

How the scoring works

A decision is worth 0, 1 or 2 points depending on the thinking behind it, and the result plays no part. The portfolio value and the score are separate. A heavy growth allocation can finish rich or poor depending on the moves, and it earns the same points either way if you handled it by the rule. Patience scores well. So does buying what fell, when the band says so.

A worked example of the band

The first rally needs nothing from you. The second one does, and the sale is sized by arithmetic.

Read after playing

Check how much of your real portfolio sits in one holding with the position concentration calculator, including stocks that show up in more than one fund. The course Mise en Place for Money sets up targets and sell rules before the money goes in, and its lesson on rebalancing rules and sell rules goes further into bands versus calendars. For the sleeve that is supposed to hold up in a scare, read the entry on defensive stocks, which explains why steady demand for food, household goods and electricity tends to keep their earnings, and so their share prices, from falling as far as the rest of the market when the economy weakens.

Score

0 / 18

Decisions made

0 / 9

Portfolio value

$100,000

Build the portfolio

Split a hypothetical $100,000 across the sleeves in steps of 5%. The total has to come to 100% before you can lock it in.

25%
15%
15%
15%
15%
15%

Total: 100%

  1. Round 1 of 6

    Rates go up

    The central bank raises its policy rate by more than markets expected. Growth stocks, priced on profits far in the future, take the hardest hit, and banks gain on wider lending margins. The moves below are made up for the game. Your plan: rebalance any sleeve whose weight is off target by 5 points or more.

    What happens to each sleeve in this round (hypothetical)

    • Large US companies-4%
    • Growth and technology-9%
    • Banks and financials+3%
    • Staples and utilities-3%
    • Energy producers+1%
    • Short-term bonds and cash-1%
    Growth and technology just fell 9%. What do you do?
    How each option scores
    • A, Weak reasoning (0 of 2). Selling after the drop locks in the loss and gives up the reason you owned the sleeve. One rate decision leaves your time horizon where it was, and the plan has no rule for selling on a bad day.
    • B, Best reasoning (2 of 2). This is the plan. A 9% fall in one sleeve often moves its weight by only a point or two, so the check may end with no trade at all, which is a fine result.
    • C, Weak reasoning (0 of 2). Buying the sleeve that just went up chases the last move. The plan buys what has drifted below target, and banks rising takes them further above it.
    • D, Defensible (1 of 2). Holding avoids a panic trade, which counts for something. Skipping the check altogether lets drift build unseen, and the plan asks you to look.

  2. Round 2 of 6

    A giant misses

    A large hypothetical technology company, one of the biggest holdings in both the large-company and growth sleeves, misses earnings and cuts its guidance. Its shares drop hard and drag its peers along. These moves are invented. Headlines declare the tech boom over.

    What happens to each sleeve in this round (hypothetical)

    • Large US companies-6%
    • Growth and technology-18%
    • Banks and financials-2%
    • Staples and utilities+1%
    • Energy producers-1%
    • Short-term bonds and cash+1%
    Suppose growth and technology has now drifted 5 points below target. What do you do?
    How each option scores
    • A, Best reasoning (2 of 2). Rebalancing after a fall means buying what dropped, and that is uncomfortable by design. You sell what held up and buy what got cheaper, back to the weights you picked while calm.
    • B, Weak reasoning (0 of 2). A headline after an 18% drop is the market describing the move it just made. Selling on it is panic selling with a reason attached.
    • C, Defensible (1 of 2). Waiting avoids a panic sale. It is still a timing bet, and the drift rule already tells you when to act.
    The same company sits in two of your sleeves. What does that tell you?
    How each option scores
    • A, Weak reasoning (0 of 2). Sleeves with different names can hold the same stocks. A large company held in a broad fund and in a growth fund is one exposure counted twice.
    • B, Best reasoning (2 of 2). Overlap is hidden concentration. With 30% in growth and 40% in large US, and the company at 10% of growth and 6% of large US, you hold 3% plus 2.4%, which is 5.4% of the portfolio in one stock.
    • C, Defensible (1 of 2). Cutting overlap is a fair aim. Selling a whole broad sleeve to get there throws away the rest of its spread, and trimming the growth sleeve, or living with a known overlap, costs less.

  3. Round 3 of 6

    Banks under pressure

    A hypothetical regional lender fails after heavy losses on its bond holdings, and worry spreads to every bank. Depositors pull money, bank shares slide for days and the rest of the market slips with them. The moves here are made up.

    What happens to each sleeve in this round (hypothetical)

    • Large US companies-5%
    • Growth and technology-3%
    • Banks and financials-25%
    • Staples and utilities+2%
    • Energy producers-4%
    • Short-term bonds and cash+1%
    Suppose the banks sleeve is now more than 5 points below its target. What now?
    How each option scores
    • A, Weak reasoning (0 of 2). This sells after a 25% fall, at the point of greatest fear. If you no longer want banks in the plan, change the plan on a calm day and move out gradually.
    • B, Defensible (1 of 2). Holding avoids selling into panic. The drift rule has been triggered, though, and doing nothing leaves the portfolio short of the weights you chose.
    • C, Best reasoning (2 of 2). The plan is doing its job by buying the sleeve that fell. Banks may keep falling after you buy, and the decision still scores 2, because it follows a rule you set and sizes the purchase by the target alone.
    • D, Weak reasoning (0 of 2). Doubling a sleeve on a hunch about a bounce is a bet on the next move. It takes one sector far past any target you wrote down.

  4. Round 4 of 6

    Oil spikes

    A supply shock pushes the oil price sharply higher within a few weeks. Energy producers rally, while companies that burn fuel or depend on consumer spending slip. The moves are invented for the game. Energy is suddenly the portfolio's best performer, and everyone seems to be talking about it.

    What happens to each sleeve in this round (hypothetical)

    • Large US companies-3%
    • Growth and technology-5%
    • Banks and financials-1%
    • Staples and utilities-2%
    • Energy producers+22%
    • Short-term bonds and cash0%
    Energy is up 22%. What do you do?
    How each option scores
    • A, Weak reasoning (0 of 2). Adding after a 22% jump chases the move. Commodity rallies can reverse as fast as they arrive, and the plan buys sleeves below target.
    • B, Defensible (1 of 2). Letting it run avoids chasing, and inside the band it matches the plan. Past the band, holding lets one sector grow into a concentration you never chose.
    • C, Best reasoning (2 of 2). The drift rule decides. At a 20% target, a 22% rise while everything else falls 3% takes energy to 24.4 of 102, about 23.9%. That is inside the band, so the rule says hold; a bigger drift would say trim.

  5. Round 5 of 6

    Recession scare

    A run of weak hypothetical economic releases, with falling orders, rising jobless claims and softer consumer spending, has headlines calling a recession. Nearly everything falls, and short-term bonds edge up as money looks for safety. These moves are made up.

    What happens to each sleeve in this round (hypothetical)

    • Large US companies-12%
    • Growth and technology-20%
    • Banks and financials-18%
    • Staples and utilities-4%
    • Energy producers-15%
    • Short-term bonds and cash+3%
    How do you respond to the drop?
    How each option scores
    • A, Weak reasoning (0 of 2). The all-clear is only obvious after prices have recovered. Selling everything here turns a drop you could hold through into a loss you have to buy your way out of.
    • B, Defensible (1 of 2). Looking away prevents a panic sale, and that is worth something. It also skips the rebalance the plan calls for once sleeves drift.
    • C, Best reasoning (2 of 2). Here the bonds and cash sleeve earns its place. Rebalancing turns the part that held up into buying power for what fell, following a rule set before the fear arrived.
    Staples and utilities fell 4%. What is the right read?
    How each option scores
    • A, Weak reasoning (0 of 2). Defensive means less sensitive to the economy. Losing money in a broad selloff is normal for any stock sleeve.
    • B, Best reasoning (2 of 2). A 4% fall against drops of 12% to 20% elsewhere is what the sleeve is for. It cushioned the portfolio and left you something to rebalance with.
    • C, Weak reasoning (0 of 2). Rebuilding the whole portfolio around the sleeve that fell least chases the last round. In the next recovery the same sleeve is likely to lag.

  6. Round 6 of 6

    Relief rally

    The feared recession fails to show up in the next batch of data. Stocks rally hard, led by the sleeves that fell most. The moves below are invented. Headlines now say the worst is over, and growth stocks are having their best stretch in a while.

    What happens to each sleeve in this round (hypothetical)

    • Large US companies+10%
    • Growth and technology+18%
    • Banks and financials+14%
    • Staples and utilities+3%
    • Energy producers+8%
    • Short-term bonds and cash0%
    Growth and technology jumped 18%. What do you do?
    How each option scores
    • A, Weak reasoning (0 of 2). Emptying the safe sleeve after a big rally chases the move and leaves nothing to rebalance with next time.
    • B, Weak reasoning (0 of 2). Locking in a gain by dumping a sleeve is a forecast that the rally is finished. The plan sells only the excess above target.
    • C, Defensible (1 of 2). Holding avoids chasing. Skipping the drift check lets winners grow past your targets, which is how a balanced portfolio slowly turns concentrated.
    • D, Best reasoning (2 of 2). Same rule, opposite direction. After a rally the rebalance sells some of what rose and tops up what lagged, so the portfolio goes into the next shock with the weights you chose.
    Looking back, which result says you played well?
    How each option scores
    • A, Weak reasoning (0 of 2). The ending value comes from the made-up moves and the allocation you picked before any of them happened. It says little about the decisions.
    • B, Best reasoning (2 of 2). Consistency is the skill being trained. A rule followed through a scare and a rally is one you can trust with real money.
    • C, Weak reasoning (0 of 2). Any diversified portfolio loses in a broad selloff. Avoiding every loss would have meant holding only cash, which carries its own cost over time.

People also ask

How do you stress test a stock portfolio?

Write down each holding's weight, then apply a set of made-up shocks, such as a sharp fall in technology stocks or a slide in banks, and add up what the portfolio would lose in each. The useful output is the loss you would face and whether you could hold through it without selling at the bottom.

How often should you rebalance a portfolio?

Many investors use either a calendar, such as once a year, or a drift band, such as rebalancing when any holding moves 5 percentage points from its target. A band acts only when drift is large enough to matter, which keeps trading and taxes down in quiet years. Situations differ, so check the tax cost before selling in a taxable account.

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