The McNamara fallacy, named for Robert McNamara, the United States Secretary of Defense from 1961 to 1968, involves making a decision based solely on quantitative observations and ignoring all others. The reason given is often that these other observations cannot be proven. (See the example below.)
It refers to McNamara's belief as to what led the United States to defeat in the Vietnam War—specifically, his quantification of success in the war (e.g. in terms of enemy body count), ignoring other variables.
The first step is to measure whatever can be easily measured. This is OK as far as it goes. The second step is to disregard that which can't be easily measured or to give it an arbitrary quantitative value. This is artificial and misleading. The third step is to presume that what can't be measured easily really isn't important. This is blindness. The fourth step is to say that what can't be easily measured really doesn't exist. This is suicide.—Daniel Yankelovich "Corporate Priorities: A continuing study of the new demands on business." (1972)
Ted has a lot of money. Lots of money makes a person happy. Ted says that he is depressed. What Ted says doesn't necessarily indicate how he feels. Depression cannot be proven. Therefore, Ted is happy.
In modern clinical trials
There has been increasing discussion of the McNamara fallacy in medical literature. In particular, the McNamara fallacy is invoked to described the futility of 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.
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