What an accuracy number is worth
Less than it looks. A single percentage collapses every kind of mistake into one figure, and the kinds are not equivalent: answering a major 3rd as a minor 3rd is a near miss on the way to being solved, while answering it as a perfect 5th means the round was above you and you were guessing. Two people at seventy per cent can be in completely different places, and only one of them should move up a level.
Accuracy is also unstable at small round counts. Twenty rounds is a mood; two hundred is a measurement. Treat a session’s number as noise and the week’s trend as signal.
Why the confusion matrix is the better record
A confusion matrix answers the question that actually matters: when you are wrong, what do you say instead? It is a grid — every answer you were given against every answer you offered — and the useful information is off the diagonal. Tight clusters just beside the correct answer mean the level is working: the errors have collapsed onto the near misses, and those are drillable directly. Scatter across unrelated answers means the level is too hard and you are picking names at random.
That is why every leaf on this site publishes a computed confusion set — the neighbouring distances for an interval, the qualities one moved tone away for a chord, the rotations and reorderings for a loop. Your matrix tells you which of those pairs is yours; the leaf page tells you what to run it against.
Streaks, spacing, and what to ignore
A streak counter measures attendance, not skill, and it is very good at turning a habit into an obligation you eventually resent. Use it if it helps you show up and ignore it entirely the week it starts making you do a token round at midnight. What genuinely predicts improvement is spacing — several short sessions across a week rather than one long one — which is the whole subject of how often to drill.
Where this data lives
In this browser, and nowhere else. There is no account, no server, and no sync: the record is written to local storage by the drill pages themselves. Clearing site data clears your history, and a different browser or device starts fresh. That is a deliberate trade — the record is less portable and it is also nobody else’s business.