“The first rule in decision-making is that one does not make a decision unless there is disagreement.”
Drucker’s main point is that strategic decisions are best made, not on the facts (which anyone can find to support a view) but through knowledge and testing of the various opinions on a situation.
To get adequate, well-rounded knowledge on a matter, an executive must have disagreement. Only disagreement and debate will provide the imagination and discovery of genuine alternative approaches. Then, to test for the best approach, an executive must determine “the criterion of relevance” by asking what evidence is necessary for reaching a decision and establishing the right measurement for ascertaining it.
Changing situations and events require new measurements of understanding what is relevant. Drucker gave the example of Secretary of Defense Robert McNamara’s reconceptualization of the measures of what to acquire so that leaders could focus on what mattered.
(The Effective Executive, chapter 7)