Achieving N policy targets generally requires at least N instruments.
Tinbergen's 1952 rule. A central bank with one instrument cannot target both inflation and employment independently, which is why dual mandates require either more instruments or an accepted trade-off.
In practice
- A council with one budget cannot cut the precept and expand a service at once, whatever the manifesto promised.
- A manager with one lever — headcount — cannot simultaneously hit a cost target and a delivery date, and will be judged on both.