Which of the following is NOT one of Taylor's four key concepts?

In the context of Taylor's principles of scientific management, the focus is on optimizing efficiency and productivity through specific methodologies. The concept of hiring a consultant for every task does not align with Taylor's four key concepts, which are centered around analyzing tasks and improving work processes.

Taylor's approach emphasizes understanding the specific elements of a task, ensuring that workers are selected, trained, and developed to perform their duties effectively, and dividing the work between management and workers to streamline operations. These key concepts are all about systematic analysis and worker empowerment to enhance productivity, rather than relying on external consultants for task management. Therefore, the idea of hiring a consultant for every task stands out as inconsistent with the core principles of scientific management outlined by Taylor.