Goodhart's Law

When a measure becomes a target, it ceases to be a good measure.

Metrics are proxies. A number approximates something deeper (health, productivity, safety); it isn’t the thing itself.

People game targets. When rewards or evaluations depend on a metric, people optimise for the number, often at the expense of the real goal. Campbell’s law and the “cobra effect” are cousins of the same idea.

What it looks like in engineering

I once sat through quarterly team “maturity” assessments: hours of whole-team scoring that didn’t end until the scores looked right, even when nothing had changed since the last quarter. Meanwhile teams were reshuffled to hit an ideal team size, which caused real friction that the scores never captured. The process measured itself, not the teams.

Using metrics well

• Measure systems and teams, never individuals.
• Use metrics to ask questions, not to hand out rewards.
• Read several together, so improving one at the expense of another shows up.
• DORA Metrics are a good example: brilliant for learning about delivery, dangerous as targets.

Sources: Goodhart’s law (Wikipedia) · Goodhart, Campbell and the cobra effect (Psych Safety)