The McNamara fallacy
Deciding only on what's measurable, ignoring the rest, until the unmeasurable is assumed not to exist.
Named for Robert McNamara, US Secretary of Defense during Vietnam, who became famous for trying to manage the war with statistics. Body counts, kill ratios, weapons captured. The numbers were measurable. They didn't capture whether the war was being won.
The pattern: in any complex situation, some things are easy to measure (revenue, headcount, hours) and others are hard (culture, judgment, customer satisfaction). The fallacy is to make decisions only on the measurable ones, then to gradually treat the unmeasurable as if it doesn't exist.
For operators, the warning is to actively account for what isn't in the data. Some of the most important factors in any business resist quantification. Ignoring them because they're hard to measure produces increasingly wrong decisions.
Examples in the wild
Companies that manage purely by KPIs often miss what's happening in their cultures, customer relationships, and competitive position. The dashboard is comprehensive about the measurable and silent about everything else.
Quant funds explicitly ignore unmeasurable factors. They often miss qualitative shifts (management quality, regulatory risk) that fundamentals investors catch.
Health metrics (weight, steps, calories) are easy to measure. Sleep quality, stress, social connection are harder. Optimising purely the measurable misses what matters.
The McNamara fallacy is one of the mental models we apply through real cases inside the Pareto MBA — a part-time program for professionals who want to think clearly about business.