In the Vietnam War, the American military treated the enemy body count as a “precise and objective measure of success.” In 1962, Brigadier General Edward Lansdale reportedly told Defense Secretary Robert McNamara to add an “x-factor” to his list of measures: the feelings of rural Vietnamese. McNamara reportedly erased it, because he could not measure it.
The public-opinion researcher Daniel Yankelovich later described the reasoning in four steps. “The first step is to measure whatever can be easily measured.” The last: “The fourth step is to say that what can’t be easily measured really doesn’t exist. This is suicide.” The error is now called the McNamara fallacy.