---
type: "Concept"
title: "Non-IID data"
description: "Heterogeneity is intrinsic, not incidental, in federated settings."
tags: [fl, heterogeneity, federated]
generated: { by: human:crpage, at: 2026-07-09T09:44:00Z }
status: stable
---

Data across clients is not independently and identically distributed; heterogeneity is a **structural condition**, not noise. Any architecture assuming IID data or uniform device availability will underperform in realistic settings, motivating clustering, personalisation and heterogeneity-aware methods.

Relates to: [Federated learning](federated-learning.md), [Kairouz: advances and open problems](../research/kairouz-open-problems.md).