One Principle, One Proof: Why IVP-Compliant Modules Minimize Change Impact

You refactor authentication. You touch UserController , LoginService , SessionManager , and APIGateway . Four files for one concern. You already know this is wrong — but can you prove it? This article takes a single, universally understood quality metric — change impact — and proves that applying the Independent Variation Principle (IVP) reduces it to the theoretical minimum. No hand-waving. No "it depends." A clean, short proof. The metric: Change Impact Every developer intuitively tracks change impact: how many modules do I have to touch when a requirement changes? Let's make it precise. A change driver is a single axis of anticipated change — a requirement, a business rule, a technology decision — that, when it changes, forces modifications to the code. We write γ for a driver. Change impact counts the modules affected when driver γ activates: impact(γ, M) = |{ M ∈ M : γ ∈ Γ(M) }| where Γ(M) is the set of drivers whose elements live in module M . This is the simplest quality metric