How to build an investment thesis

An investment thesis is a short written argument for owning a stock: what the business is, the two or three things that have to go right, what it's worth against what you pay, and the specific events that would prove you wrong. To build one, start from the business rather than the price, check the story against the company's filings, write your exit conditions before you buy, and fit it on one page. Then revisit it every quarter. This guide walks through seven steps and fills in a one-page thesis for Costco with numbers from its 10-K filings.

This is a research framework, not investment advice. Investi is a research and information tool, not a licensed investment adviser, and nothing here is a personal recommendation.

#What is an investment thesis?

An investment thesis is a falsifiable argument about the future of a business, and about why the market may be mispricing it today. Each part of that definition matters:

• It's an argument, not a fact. You are making a claim about an uncertain future, so you say in advance how you would know you were wrong.
• It's about a business, not a ticker or a chart. A share is a claim on a company's future cash flows. If you can't describe the business plainly, you don't have a thesis yet.
• It says why the market may be wrong. Buying a stock implies you see something not fully priced in: durability it underrates, growth it doubts, a risk it overweights. If the market already agrees with you, the good news is in the price.

Writing it down is the point. Memory edits itself to match the price. When a stock falls, an investor without a written thesis tends to find new reasons to hold; when it rises, they forget they ever doubted it. The document keeps you honest with your future self.

#What does a finished thesis answer?

A complete thesis answers six questions:

1. What is the business, in a sentence or two a non-expert would understand?
2. How does it make money, and why does that last?
3. What two or three things have to go right?
4. What is it worth, and what am I paying?
5. What would prove me wrong?
6. Why do I see this differently from the market?

The seven steps below build each answer in turn.

#Step 1: Start with the business, not the stock

The most common mistake is starting from the ticker (the price, the chart, the recent move) and working backward to a reason to buy. Start instead from the plainest description of what the company does.

Write one sentence a twelve-year-old would understand. "They sell software that companies use to run payroll." "They own toll roads and collect a fee from every car." If you can't do this without jargon, you don't understand the business well enough to own it yet. That is useful to know.

#Step 2: Understand how the company makes money

Now trace the flow of money. Who pays the company, for what, how often, and why do they keep paying?

This is where you find durability. A business that has to win every customer again each year is fragile. A business customers can't easily leave, or wouldn't want to, has a moat. Ask:

• Where does revenue come from: one product or many, one large customer or thousands?
• Is it recurring or one-off? Subscriptions and consumables tend to be steadier than one-time sales.
• What stops a competitor from taking it: scale, a network effect, switching costs, a brand, a patent, a license, the lowest costs?
• Who has the pricing power: the company, its customers or its suppliers?

Name the moat specifically, or admit there isn't one.

#Step 3: Find the two or three things that actually matter

Every business has dozens of moving parts, and few of them decide the outcome. The skill is finding those few and ignoring the rest.

For a subscription software company it might be net revenue retention, the cost of acquiring a customer, and how long customers stay. For a retailer, same-store sales growth and gross margin. For a commodity producer, the commodity price and its cost per unit.

Once you know your drivers, your research has a spine. You stop following every headline and watch the specific numbers that would confirm or break your case.

#Step 4: Check the numbers against the story

A thesis is a story, and the financial statements are where you find out whether it's true. You don't need a finance degree. You need to check whether the numbers agree with the argument:

• Growth: is revenue growing the way the story implies, and is it speeding up or slowing?
• Profitability: are margins stable, or moving the way the story predicts?
• Cash: does the business generate cash, or only accounting profit? Free cash flow is harder to flatter than earnings. Heavy capital spending is where the two split, as what Microsoft, Alphabet, Amazon and Meta spend on capex shows.
• The balance sheet: how much debt, and could it survive a bad year without raising money on bad terms?
• Returns on capital: when the company reinvests a dollar, does it earn a good return on it?

Take Costco. Its story is that it sells goods at close to cost and earns its profit on membership fees. If that's true, the margin on merchandise should be thin and steady. The fiscal 2026 10-K reports $297 billion of net sales and $264 billion of merchandise costs, which leaves 11.1 cents of gross profit per dollar of goods sold. Over the decade from fiscal 2016, that margin stayed between 10.5% and 11.3%.

The numbers corroborate the story. When they contradict it, the numbers are usually right. Ground each claim in a figure you can pull and point to, from the latest filing: a number recalled from memory may be a year old.

#Step 5: Decide what it is worth, and what you are paying

A wonderful business at the wrong price can still be a bad investment. Valuation connects the story to a number.

You don't need a fifteen-tab model. You need a defensible view of what the business is worth and an honest look at the gap to today's price. Anchor it in something real: what are you paying for each dollar of earnings or free cash flow, and is that reasonable given how fast and how durably those earnings grow?

Then write the gap down: "I think this is worth roughly X. It trades at Y. The difference is my margin of safety." If there's no gap, and you'd only make money if someone later pays even more, you have a momentum bet rather than a value thesis. That can be a strategy, but be clear about which one you're running.

#Step 6: Write down what would prove you wrong

This is the step most people skip. Before you buy, write the specific, observable events that would tell you the thesis is broken.

A falling price isn't one of them. The real triggers are things like: growth falls below a level for two straight quarters, the moat visibly erodes, margins fall, a key driver reverses, or management does something that contradicts the reason you trusted it.

These are your kill criteria. They make the sell decision in advance, in a calm moment, so that when the moment comes, noisy and emotional, you follow a rule you already set instead of negotiating with yourself.

#Step 7: Fit it on one page

Put the answers on a single page. If they don't fit, the weak parts of the argument are probably hiding in the length. Copy this template, keep the headings, and fill the blanks. A line that stays empty is the part you haven't thought through.

Company.

Date.

The business, in a sentence.

How it makes money, and why that lasts.

The two or three drivers I am betting on.

What it is worth versus what I am paying.

Kill criteria. The events that would make me sell. Price is not one of them.

My edge. Why I see this differently from the market.

#What does a finished thesis look like?

This is a filled page for Costco, written in October 2026 after its fiscal 2026 10-K. It is not a recommendation to buy or sell. It shows what the page looks like when someone has finished thinking.

Company. Costco Wholesale (COST)

Date. October 2026, after the 10-K filed October 7, 2026.

The business, in a sentence. A membership warehouse: people pay each year for the right to buy goods in bulk, near cost.

How it makes money, and why that lasts. Merchandise is priced to move: about 11 cents of gross profit per dollar of sales, as Step 4 showed. The fee is where the profit concentrates. Membership fees were $5.91 billion in fiscal 2026, about half of the $11.7 billion operating income. Scale buying keeps prices low, low prices keep members renewing, and renewals keep the scale. That loop is the moat.

The two or three drivers I am betting on. The renewal rate and membership growth. Comparable sales in the core warehouse, excluding gas. Room to raise the fee without hurting renewals.

What it is worth versus what I am paying. The question is what you pay for a fee stream that has grown every year for a decade. If the price only makes sense for a high-quality company at a full multiple, there may be no gap. Then you don't have a value thesis; you have a quality company at a full price, and those are different things.

Kill criteria. The renewal rate falls two years in a row. A fee increase that costs members and doesn't win them back. Comparable sales excluding gas turn negative for several quarters. Costco starts chasing merchandise margin the way a typical retailer would, which would break the loop.

My edge. I shop there, and I can see how a household plans around the card. That's a small edge, not a secret model. If the only thing I can write here is that everyone knows Costco is well run, I don't have an edge. I have the consensus.

The page doesn't say buy. It says what has to stay true, and what would end the argument.

#What does checking a kill criterion look like?

Kill criteria only help if you check them. Costco reports its renewal rate in Item 1 of each 10-K. Worldwide, it was 90.5% at the end of fiscal 2024 (10-K), 89.8% in fiscal 2025 (10-K) and 89.8% in fiscal 2026 (10-K). In the U.S. and Canada it went from 92.9% to 92.3%, then stayed at 92.3%.

Against the criterion "the renewal rate falls two years in a row", that's one decline followed by a flat year, so it hasn't triggered. It is the line to read first in the next 10-K. Before comparing years, read how Costco defines the rate in the same filing: it describes it as a trailing calculation, and a change in definition can move the number without a change in the business.

#Where do most theses fail?

The same patterns show up again and again:

• A story with no falsifier. If nothing could prove it wrong, it's faith, not analysis.
• Confusing a good company with a good investment. The price is half the thesis.
• Anchoring to the purchase price. The only question is whether the thesis is still true from here.
• Researching endlessly to avoid the decision. The drivers are few; find them and decide.
• Never revisiting it, which is the most common failure of all.

#How do you keep a thesis alive after you buy?

Writing a thesis is the easier half. Every quarter brings new evidence: results that confirm or contradict your drivers, competitive moves that test the moat, decisions by management. The habit that matters is checking the thesis against that evidence on a schedule.

Few investors do this, often because their notes live in a document that never gets reopened. Investi keeps your thesis next to the company's filings. The analyst can open what you wrote last quarter and check it against the latest numbers, citing each one to its filing, and when it helps revise your notes it proposes the edit for you to approve. A routine can rerun a question like "check my Costco kill criteria against the latest filings" on a schedule and email you the result. How the Investi analyst works explains the design.

The tool is secondary, though. A thesis you wrote down, kill criteria you set in advance, and a standing appointment to check both are most of the discipline, and they work in a paper notebook too.

#How can you check this yourself?

Every Costco number in this guide comes from its 10-K filings on SEC EDGAR. To check them, search for Costco (ticker COST) on EDGAR, open the 10-K filed October 7, 2026, and look at three places:

1. The income statement, for net sales, membership fees, merchandise costs and operating income.
2. Item 1, Business, for the renewal rate sentence and how the rate is calculated.
3. The earlier 10-Ks, for the same lines in past years.

Or ask Investi the question and open the citations: each figure links to the line of the filing it came from.

The fastest way to learn this is to write one. Pick a company you already understand, fill in the page, and force it onto one sheet. If you want to keep it in Investi, sign in, add the company, and ask the analyst to check your drivers against the latest filings.