The McNamara fallacy refers to Robert McNamara

Robert McNamara

Robert Strange McNamara was an American business executive and the eighth Secretary of Defense, serving under Presidents John F. Kennedy and Lyndon B. Johnson from 1961 to 1968, during which time he played a large role in escalating the United States involvement in the Vietnam War...

, the United States Secretary of Defense

United States Secretary of Defense

The Secretary of Defense is the head and chief executive officer of the Department of Defense of the United States of America. This position corresponds to what is generally known as a Defense Minister in other countries...

 from 1961 to 1968, and his belief as to what led the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 to defeat in the Vietnam War

Vietnam War

The Vietnam War was a Cold War-era military conflict that occurred in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. This war followed the First Indochina War and was fought between North Vietnam, supported by its communist allies, and the government of...

. It refers to the quantifying of success in the war (e.g. in terms of enemy body count

Body count

A body count is the total number of people killed in a particular event. In combat, a body count is often based on the number of confirmed kills, but occasionally only an estimate.-Military use:...

) while ignoring other variables.

Examples

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.