McNamara Fallacy

Named after Robert McNamara's Vietnam War strategy of measuring success by body counts: the error of making decisions based solely on quantitative metrics while ignoring qualitative factors that are harder to measure but equally or more important. The fallacy has four steps: (1) Measure whatever is easily measured. (2) Disregard what can't be easily measured. (3) Presume what can't be measured isn't important. (4) Presume what can't be measured doesn't exist. In business: measuring employee performance by easily tracked output while ignoring mentoring, culture-building, and creative thinking that can't be quantified.

When to use it

When dashboard metrics are improving but reality doesn't feel better; when decisions are being driven entirely by quantifiable factors while qualitative factors are ignored; when 'what gets measured gets managed' is producing management of the wrong things; when important outcomes aren't captured by existing metrics.

How it can help

For any important decision: ask 'what's important here that we're NOT measuring?' Then give explicit weight to unmeasured factors alongside measured ones. The practice: maintain a 'dark matter' list—important factors that influence outcomes but don't appear in any dashboard. Customer trust, team morale, code quality, institutional knowledge, and relationship depth are all dark matter that the McNamara Fallacy discounts to zero. In performance evaluation: explicitly evaluate qualitative contributions alongside quantitative metrics, even if qualitative assessment requires more judgment and effort.