Final answer:
The concept that describes a fixed percent improvement in unit performance each time the total production quantity doubles is known as the "rule of 70".
Step-by-step explanation:
The concept that describes a fixed percent improvement in unit performance each time the total production quantity doubles is known as the "rule of 70." The rule of 70 states that the time it will take for a system or collection to double in size is 70 divided by the percentage growth rate. This concept is primarily used in mathematics and economics to study exponential growth and compound interest.