Reading a Form 4: Which Insider Trades Mean Anything
Most filed transactions are not decisions. The code letter tells you which ones are — and open-market buying is under a tenth of the total.
When a corporate officer, director or 10% owner trades their own company's stock, they must report it to the SEC on Form 4, generally within two business days. The filings are public, free, and widely misread — because most of what appears in them is not a decision anyone made about the company's prospects.
The code letter is the whole thing
Every transaction on a Form 4 carries a one-letter code. It is the difference between a signal and an administrative event, and it is the field most coverage ignores. Across the filings held here:
| Code | Meaning | Transactions | Share |
|---|---|---|---|
| S | sale | 15,716 | 45.1% |
| A | grant | 4,671 | 13.4% |
| M | option exercise | 4,381 | 12.6% |
| F | tax withholding | 4,123 | 11.8% |
| P | purchase | 2,994 | 8.6% |
| J | other | 868 | 2.5% |
From 34,839 Form 4 transactions across 3,172 tickers.
What each one actually is
- P — open-market purchase. The insider decided to buy, at market price, with their own money. This is the one worth attention, and it is a small minority of filings.
- S — sale. The largest category by far, and the weakest signal. Executives are paid in stock and sell to diversify, buy houses and pay for things. A sale means someone needed money at least as often as it means anything else.
- A — grant or award. The company handed over shares as compensation. No decision by the insider at all.
- M — option exercise. Converting options into shares, usually because they are expiring. Frequently paired with an immediate sale, which then shows up as an S and gets reported as insider selling.
- F — shares withheld for tax. The company keeps some shares to cover withholding when an award vests. Entirely mechanical, and it appears as a disposal.
Add A, M and F together and a large share of all "insider activity" is compensation machinery. A headline counting transactions without filtering by code is describing payroll.
10b5-1 plans
Insiders can adopt a written plan that schedules trades in advance, so they execute automatically while the insider is in possession of information they cannot trade on. A sale under such a plan was decided months earlier and tells you nothing about today.
Form 4 has a checkbox for this and a field for the plan's adoption date, which is the useful part — a sale under a plan adopted three weeks before a collapse is a different object from one under a plan adopted two years ago. Rules tightened in 2023 to impose cooling-off periods, which makes recent filings more informative than older ones.
Why purchases are treated as the signal
The asymmetry is the standard argument and it holds up reasonably. There are many innocuous reasons to sell and essentially one reason to buy more of something you are already heavily exposed to: you think it is cheap. An executive whose salary, bonus, options and existing holdings all depend on one company is already concentrated, and adding more is a deliberate act against diversification.
Two refinements worth applying. Size relative to the insider's holdings matters more than the dollar amount — a director buying 5% more of their position says more than one buying a token amount. And cluster buying, several insiders at the same company buying within a short window, is harder to explain away than any single purchase.
The limits
- Two days of delay, minimum. Anything fast-moving has already moved.
- Insiders are wrong regularly. Conviction about your own company is not the same as being right about it, and executives are subject to the same optimism about their work as anyone else.
- Small companies, thin data. One director's purchase in a microcap is one person's opinion.
- Free-text ticker fields. Filers type the symbol by hand, so the raw data contains entries like "NONE", "N/A" and exchange-prefixed symbols. Anything aggregating Form 4 data has to handle that or it produces nonsense.
Reading them here
The insider trading pages carry every parsed transaction with its code, the filer, the date and a link to the original filing on EDGAR — parsed from the SEC's own archive rather than bought from a data vendor. Filtering to code P is the first thing to do with it.
Figures on this page are counted from the SEC Form 4 filings held in this site's database and are updated nightly. Reference material, not investment advice.
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