mirror of
https://github.com/MBombeck/HealthLog.git
synced 2026-11-14 14:40:52 +00:00
Marc-André Bombeck
ac86199808
About a dozen hand-rolled spreads each picked population or sample variance without saying so, and the two surfaces a person compares picked differently: the series API reported the population SD of a blood-pressure series, the MCP tools and the AI payload the sample SD of the same readings, about 22 % apart at three readings. `lib/stats/spread` holds `variance` and `standardDeviation` with an explicit `population | sample` kind and `READING_SPREAD`, the kind every surface uses to report the spread of a person's readings. The series route and the feature payload both report it; the spreads that feed a verdict (the normal-swing checks, the changepoint threshold) and the clinical glucose SD stay the sample estimate, now said out loud. A guard fails on a new hand-rolled spread and on the two surfaces drifting apart.
50 lines
1.7 KiB
TypeScript
50 lines
1.7 KiB
TypeScript
/**
|
||
* Variance and standard deviation, with the kind named at every call site.
|
||
*
|
||
* - `population` divides by n: the spread of exactly these readings. It is
|
||
* what a surface reports when it describes a series the person can see.
|
||
* - `sample` divides by n − 1: an estimate of the spread that readings like
|
||
* these come from. It belongs where the spread feeds a decision (a
|
||
* significance verdict, a changepoint threshold) or where a clinical
|
||
* definition asks for it.
|
||
*
|
||
* The two differ by sqrt(n / (n − 1)), about 22 % at n = 3, so one series
|
||
* shown with both rules looks like two different series. Every hand-rolled
|
||
* spread used to pick its rule silently; the API reported the population SD
|
||
* of a blood-pressure series while the MCP and AI payload reported the
|
||
* sample SD of the same readings.
|
||
*/
|
||
|
||
export type VarianceKind = "population" | "sample";
|
||
|
||
/**
|
||
* The kind every surface uses to report the spread of a person's readings,
|
||
* so the API, the MCP tools, the Coach and the AI payload say the same
|
||
* number for the same series.
|
||
*/
|
||
export const READING_SPREAD: VarianceKind = "population";
|
||
|
||
/** `null` when the kind is undefined for this many values (n < 1, or n < 2 for a sample). */
|
||
export function variance(
|
||
values: readonly number[],
|
||
kind: VarianceKind,
|
||
): number | null {
|
||
const n = values.length;
|
||
const divisor = kind === "sample" ? n - 1 : n;
|
||
if (divisor < 1) return null;
|
||
let sum = 0;
|
||
for (const v of values) sum += v;
|
||
const mean = sum / n;
|
||
let squares = 0;
|
||
for (const v of values) squares += (v - mean) ** 2;
|
||
return squares / divisor;
|
||
}
|
||
|
||
export function standardDeviation(
|
||
values: readonly number[],
|
||
kind: VarianceKind,
|
||
): number | null {
|
||
const v = variance(values, kind);
|
||
return v === null ? null : Math.sqrt(v);
|
||
}
|