How to write an investment thesis for a stock you already own

Why the usual advice does not fit

Search for how to write an investment thesis and you will mostly find material written for venture capital and private equity — a fund’s strategy document, or an analyst’s memo arguing for an allocation. It is genuinely good writing, and it is aimed at a decision you are not making. You are not deciding whether to take a position. You took it.

That difference matters more than it sounds, because writing a thesis after the fact has a problem the pre-purchase version does not: you already know what the price has done since. Whatever you write will be contaminated by that, and pretending otherwise is the fastest way to produce a document that is useless in a year.

So the job is not to reconstruct what you thought. It is to state, as of today, what you are actually betting on — and to be honest about which parts of that you believed at the start and which you acquired along the way.

Start from the sell, not the buy

The most useful question is not “why do I own this?” It is: knowing everything I know now, would I buy this today at this price?

It is a better question because it cannot be answered with a story about the past. If the answer is yes, the reasons you give are your thesis, in current terms, and you can write them down without archaeology. If the answer is no, you have learned something more valuable than a thesis: you are holding something you would not buy, which is a position held by inertia rather than by argument.

The uncomfortable middle answer — “yes, but smaller” — is the most common and the most informative. It usually means the thesis is intact but your conviction has quietly drifted down while the position size stayed where it was.

The four things worth writing

Keep it short. A three-page thesis is harder to check later, not easier, because you will not remember which of its twelve points was load-bearing. One page is plenty.

1. The claim. One or two sentences, in the present tense, about the business rather than the stock. “A great company with a strong moat” is a mood. “Its switching costs let it raise prices roughly with inflation without losing units, so margin holds as it scales” is a claim: it says something that could turn out to be false.

2. What has to stay true. Two or three conditions the claim depends on. If you cannot name any, the claim is too vague to be worth holding. These are usually the things you would be genuinely surprised to see go the other way.

3. What would prove you wrong. The specific, observable condition that would mean the reason you own it is gone. This is the part people skip, and it is the part that does the work. It deserves its own treatment, which is here.

4. What you already know you are ignoring. Every position has one: the bear argument you have heard, find plausible, and have decided not to act on. Write it down along with why. In a year, when it turns out to matter, the useful question is not whether you saw it coming — you did — but whether your reason for dismissing it still stands.

A worked example

Suppose you bought a payments processor three years ago. You remember liking the network effects. The stock is up 40% and you have not thought about it properly since.

The bad version of the write-up: “Strong network effects, great management, growing market. Long-term hold.” Nothing in that can ever be checked, so nothing in it can ever tell you to sell.

The useful version:

Merchants do not switch processors for a few basis points, because integration is painful, so take-rate holds above 1.7% while volume compounds in the high teens. Depends on: no large platform bundling payments for free; developer share not eroding to a cheaper API. Would prove me wrong: take-rate below 1.5% for two consecutive quarters, or a top-five merchant publicly moving off the platform. Ignoring: the bear case that a competitor’s pricing pressure is real but its enterprise coverage is years away — if that gap closes, I am wrong.

That is four sentences, and it is checkable. Every quarter, three numbers and one piece of news settle whether it still holds. Nothing about it requires you to remember what you thought three years ago.

Doing it for a portfolio you never documented

If you have eight positions and none of them written down, do not try to do all eight in one sitting. It becomes a chore, the last four get worse than the first four, and the exercise gets abandoned.

A better order: start with the largest position, because that is where being wrong costs most. Then do the one you feel most defensive about — that reaction is usually information. Then the rest, one a week.

Do it against the most recent annual report rather than from memory. The company has told you what it thinks its business is and what it thinks the risks are; that is a far better prompt than a blank page, and it also grounds the thesis in the same documents that will later be used to check it.

Then the part that actually decides whether this was worth doing

A thesis is not a document, it is a comparison you have not made yet. Its whole value arrives later, at the moment new information lands and you check the new against the written. Most theses never reach that moment: they get written in a burst of good intentions and are never opened again, because nothing tells you when opening one would have mattered.

You can solve that manually — a calendar reminder against each position, quarterly at minimum, read the filing against what you wrote. That genuinely works, and it is free.

Or it is the job Okidor was built to do: hold the thesis you wrote, read every new filing, earnings call and material news item against it, and tell you which of your stated assumptions each one moved. You can see what that output looks like on a worked example without an account, and compared against the alternatives if you would rather decide on your own terms.

Nothing here is investment advice, and the example is illustrative rather than a recommendation. See the disclaimer.