How to keep track of your direct reports
You can hold a clear read on one or two people in your head. Somewhere around the fourth or fifth report it stops working: the picture of each person blurs into the last thing that happened, and "how is everyone doing" becomes a question you can only answer for whoever you spoke to most recently. The usual reflex is to take more meeting notes, which doesn't fix it, because meeting notes hold what was said in the room and what you're losing is the model underneath, your evolving sense of how each person works.
This is a light habit for keeping that model, without it turning into a second job or a file you'd be uncomfortable for anyone to read. If you've only just started managing, there's a page on the transition itself.
Keep the read, not the transcript
The thing worth writing down is a plain sentence about how someone works: "Sam is strongest kicking off ambiguous work and loses momentum in the long middle." It's a claim rather than a rating or a number, and specific enough that a later version of you could look at it and decide it was wrong.
Two small things go with each read. How sure you are, said in words: a hunch you'd call uncertain, something you've seen twice that's likely, a pattern you'd now call plausible or, once in a while, certain. Words, not a percentage, because a percentage on a person invites a precision you don't have and a math you shouldn't be doing. And where the read stands right now: something you're acting on, something you're only watching, something settled, or something you've let go. The point of both is to let you write a half-formed thought down honestly, instead of either inflating it to a conclusion or not writing it at all.
Catch what you notice when you notice it
Between the reads are the smaller things: a good week, a rough one-on-one, a moment in a meeting where someone stepped up. Note them as they happen, with a direction: pressure up, ownership steady, clarity down, whatever words match how you already think. The reason to catch them in the moment is that the moment is the only time you have them accurately. A note written the day it happened carries a real date. The same thing recalled at review time is a reconstruction, and a reconstruction favors whatever's recent.
Keep these lines to what happened, not what you made of it. "A design doc at half its usual depth" is something you noticed; "burning out" is a read, and it goes in as one, with how sure you are next to it. The test for a noticing line is whether a colleague who was there would agree it happened, whatever they made of it. The two also age differently: the thin doc is as true in December as the day you wrote it, while "burning out" is one of several stories that fit it; a hard problem or a slow week fits just as well.
Give the older ones permission to fade. Something you noticed once in March and never again shouldn't still be shaping your read in December at the same volume as last week. If it matters, you'll notice it again and it comes back up. What you want in front of you is what's currently live, not a running tally of everything anyone ever did.
Let the read change where you can see it change
A written read isn't fixed; it's there so you can watch it move, and keep the earlier versions when it does. When something cuts against what you believed, revise the sentence and leave the old one underneath, dated. Six months on, "how did I come to think this about them" has an answer you can actually read, instead of the tidy story you'd otherwise assemble to match wherever you ended up.
Re-read before it matters
The notes pay out at the moments the job gets hard: a review, a promotion case, a conversation you're dreading. Before any of them, re-read the person's page start to finish, oldest lines first. Fifteen minutes, and the February work that would have quietly fallen out of a review written from memory is back in it. While you're there, check the dates. A read whose newest evidence is a quarter old doesn't go in as a fact; it goes into the next one-on-one as a question. That one habit undoes most of what recency does to a review, and writing the review itself from the page is its own guide.
Keep only what helps you manage
You're writing about real people. Keep to what would belong in a conversation about their work, prune what you'd no longer stand behind, and write as if they'll read it, because in some places, including the EU and UK, they can. What belongs in the file, who can legally read it, and where it should live is its own page.
How much this actually takes
Little enough that it survives a real week, or it won't survive at all. A few lines after each one-on-one, and a quick note whenever something genuinely shifts your read on someone. If you want fields to start from, the 1:1 notes template has them, and the Notion setup is one place to keep them. A handful of people, a few minutes a week. You're not aiming for a complete record, only to walk into the next review, or the next hard conversation, reading from something instead of rebuilding it from the last month.
Do it for a couple of months and it starts giving back what memory can't: the week your read on someone turned and what turned it, or the person you'd half written off in spring and hadn't revisited since. The whole arc is there when a decision about someone's job depends on it.
More guides in Managing people over time.
Full disclosure: I make Continuum, a Mac app that runs this habit so the fiddly parts don't end it. You tag what you notice in an entry and it updates the reads it touches on its own, leaving a note about what moved; older notes fade unless you see the same thing again; and the app opens on the reads that have gone stale and the people you've gone quiet on. Its beliefs carry words, its signals carry a direction, and nothing adds up to a score. Nothing leaves your Mac. Free for up to three people; Pro is a one-time purchase if you'd rather not subscribe. The habit above works in any plain document; the app is for when keeping it by hand is the part that slips.