The McNamara fallacy (also known as the quantitative fallacy),^[1] named for Robert McNamara, the US Secretary of Defense from 1961 to 1968, involves making a decision based solely on quantitative observations (or metrics) and ignoring all others. The reason given is often that these other observations cannot be proven.

But when the McNamara discipline is applied too literally, the first step is to measure whatever can be easily measured. The second step is to disregard that which can't easily be measured or given a quantitative value. The third step is to presume that what can't be measured easily really isn't important. The fo[u]rth step is to say that what can't be easily measured really doesn't exist. This is suicide.

— Daniel Yankelovich, "Interpreting the New Life Styles", Sales Management (1971)^[2]

The quote originally referred to McNamara's ideology during the two months that he was president of Ford Motor Company, but has since been interpreted to refer to his attitudes during the Vietnam War.

The McNamara fallacy is often considered in the context of the Vietnam War, in which enemy body counts were taken to be a precise and objective measure of success. War was reduced to a mathematical model: By increasing estimated enemy deaths and minimizing one's own, victory was assured. Critics such as Jonathan Salem Baskin and Stanley Karnow noted that guerrilla warfare, widespread resistance, and inevitable inaccuracies in estimates of enemy casualties can thwart this formula.^[3][4]

US Air Force Brigadier General Edward Lansdale reportedly told McNamara, who was trying to develop a list of metrics to allow him to scientifically follow the progress of the war, that he was not considering the feelings of the common rural Vietnamese people. McNamara wrote it down on his list in pencil, then erased it and told Lansdale that he could not measure it, so it must not be important.^{[5][page needed]}

McNamara's interest in quantitative figures is also seen in Project 100,000 aka McNamara's Folly: by lowering admission standards to the military, enlistment was increased. Key to this decision was the idea that one soldier is, in the abstract, more or less equal to another, and that with the right training and superior equipment, he would factor positively in the mathematics of warfare. Inductees of the project died at three times the rate of soldiers who met the earlier standards.^[6]

Donald Rumsfeld, US Secretary of Defense under George W. Bush, sought to prosecute wars with better data, clear objectives, and achievable goals. Writes Jon Krakauer:

... the sense of urgency attached to the mission came from little more than a bureaucratic fixation on meeting arbitrary deadlines so missions could be checked off a list and tallied as 'accomplished'. This emphasis on quantification has always been a hallmark of the military, but it was carried to new heights of fatuity during Donald Rumsfeld's tenure at The Pentagon. Rumsfeld was obsessed with achieving positive 'metrics' that could be wielded to demonstrate progress in the Global War on Terror.

— Jon Krakauer, Where Men Win Glory.^[7]

There has been discussion of the McNamara fallacy in medical literature.^[8][9] In particular, the McNamara fallacy is invoked to describe the inadequacy of only using progression-free survival (PFS) as a primary endpoint in clinical trials for agents treating metastatic solid tumors simply because PFS is an endpoint which is merely measurable, while failing to capture outcomes which are more meaningful, such as overall quality of life or overall survival.

In competitive admissions processes—such as those used for graduate medical education^[10]—evaluating candidates using only numerical metrics results in ignoring non-quantifiable factors and attributes which may ultimately be more relevant to the applicant's success in the position.