McNamara Fallacy

The McNamara fallacy, also called the quantitative fallacy, is named for Robert McNamara, United States Secretary of Defense from 1961 to 1968, and describes making a decision based solely on quantitative observations or metrics while ignoring all others, often on the reasoning that other observations cannot be proven. As described by Daniel Yankelovich, the pattern proceeds in four steps: first measuring whatever can easily be measured, which is fine as far as it goes; then disregarding what cannot easily be measured, or assigning it an arbitrary quantitative value, which is artificial and misleading; then presuming that what cannot be measured easily is not very important, which is a form of blindness; and finally treating what cannot be measured as if it does not exist at all. Yankelovich argued that this reasoning was part of what led the United States to defeat in the Vietnam War, pointing specifically to McNamara's focus on quantifiable measures of success such as enemy body counts while other variables went unweighed.