# Pooled numbers hide sign flips

> A statistic pooled across segments can describe no segment that exists, and splitting it is where bias gets in.

Building block · Testing whether an edge is real · found 2026-09-23 · by John Kingola

## What I was looking for

Whether forecast ambiguity tracks error the same way across data sources.

## What I found

The pooled number gets quoted; the split number gets acted on. The first can hide segments running the other way, and the second can be a false positive from trying many splits.

In my forecasting work, a pooled correlation of −0.59 split into +0.175, −0.005, −0.266 and −0.500 by source. Because the splits were registered first, the difference between sources counts as a finding.

## The move

I register the splits before the data arrives, so the segments are a result, not a search.

Web version: https://kingola.dev/notes/pooled-numbers-hide-sign-flips/