What Is the Difference Between Precision and Accuracy?

In the demanding world of precision motion control, automation and advanced manufacturing, terminology is everything. Misunderstanding fundamental concepts like precision and accuracy can lead to flawed designs, produce costly errors and severely compromise final product quality. While these terms are often used interchangeably in everyday conversation, they define two distinct and separate metrics of quality in motion systems. 

Accuracy relates to trueness, while precision relates to consistency and resolution. Both are absolutely vital for the successful operation of technologies like precision stages which require flawless movement at microscopic scales where tolerances are measured in nanometers. For high-performance engineering to succeed, these distinctions must be crystal clear.

This comprehensive guide breaks down the core concepts of accuracy, precision repeatability and resolution, providing the clarity engineers and designers need to specify, operate and successfully integrate high-performance automation systems into their most critical processes. 

What Is Accuracy?

Accuracy is a measurement of how close a machine's actual position or movement is to the position or movement it was commanded to achieve. This represents the absolute trueness of the motion against a known standard. For example, if a stage is commanded to move to 100.000 millimeters but it consistently settles at 100.005 millimeters, the 5-micrometer difference represents the system's inaccuracy.

Achieving accuracy is a significant challenge. Absolute accuracy requires comprehensive error mapping and sophisticated software compensation to correct for systematic errors across the stage's travel range. This effort ensures the system performs reliably despite internal mechanical imperfections. Accuracy is measured with tools like a laser interferometer and often verified against standards like ISO 230-2 and similar. Factors that influence it include bearings, the motor, machine part tolerances, temperature, friction, stiffness and compliance, particulate contamination and more.

What Is Precision?

Precision relates to a combination of measurement resolution and the consistency and reliability in reproducing results. Precision is solely concerned with the scatter or spread of the measured data points, not their proximity to the target value. High precision means that when the same movement is commanded multiple times, the stage will reliably stop at virtually the identical physical coordinate (regardless of whether that coordinate is the correct one).

Precision is often quantified by the standard deviation of measurements. Factors that influence precision include backlash and hysteresis, friction, feedback resolution, control loop stability, thermal/temperature variations and more. While high resolution is required for high precision, it does not guarantee it. An encoder might resolve down to one nanometer, but if the stage mechanics have friction variations, the achieved precision will be worse. Understanding what is accuracy and precision is critical for error management.

What Is the Difference Between Precision and Accuracy?

To compare and contrast precision and accuracy with clear examples, consider the classic analogy of a dartboard where the bullseye is the target position. Accuracy is hitting the bullseye; precision is hitting the same spot every time. If all your darts land tightly grouped in the top-left corner, your system is highly precise but inaccurate.

The key functional difference lies in correctability. If something is precise and repeatable, then it can be made to be accurate through calibration. If it's not precise, then it can't be truly accurate. A highly precise stage allows engineers to create a software look-up table or error map that corrects its known systematic errors. This highlights why precision provides the necessary foundation for high absolute positioning performance.

What Is the Difference Between Accuracy and Repeatability?

Repeatability is the practical measurement used to quantify precision, specifically referring to the machine's performance over repeated movements to the same target. Repeatability is the degree to which the same output is achieved for a fixed input. In other words, it depends on how close the results or observations are to one another. Repeatability is the most frequently cited specification for motion systems.

If a stage executes a move from point A to point B 20 times, repeatability measures the diameter of the circle containing all 20 finishing positions. Accuracy measures the distance from the statistical center of that circle to the ideal point B. The close relationship between accuracy and precision means that high repeatability is essential for maximizing both.

Is Accuracy or Repeatability More Important?

When designing a high-end motion system, the question of which metric is paramount—accuracy or repeatability—is a key design decision. Repeatability is generally more important than accuracy, because if something is repeatable, it can be made to be accurate through calibration or similar means. A highly repeatable system is inherently predictable. Its errors are systematic and correctable.

In semiconductor and silicon photonics, extreme repeatability is non-negotiable for lithography and component alignment. In precision manufacturing and test and inspection, high repeatability ensures every product is manufactured under identical conditions, guaranteeing quality control. While absolute accuracy is the ultimate goal, precision (quantified by repeatability) is the indispensable enabler.

What Is Precision Engineering?

Precision engineering is a discipline encompassing the design, development and measurement of high-accuracy components and devices. This specialized field is not merely about achieving tighter tolerances; it is a holistic focus on creating components and machines that achieve motion and dimensional stability previously deemed impossible.

Tools and techniques used include advanced metrology such as laser interferometry, low-friction elements like air bearings and deterministic design principles that minimize error. Precision engineering moves beyond relying on estimation and focuses instead on ensuring that a system’s performance is guaranteed through verifiable, physics-based analysis and validation methods.

Resolution vs. Accuracy vs. Precision

In any discussion of motion control metrics, it is essential to clarify the difference between the three primary specifications. Resolution is the smallest increment of movement the control system can detect or command, defined partly by the system’s encoder or other feedback device. Accuracy is the degree of closeness to the true position. Repeatability is the consistency or spread of the measurements. Precision encompasses all of these concepts.

An encoder may have a high resolution of 1 nm, yet the stage's straightness or geometric characteristics (such as pitch, roll or yaw) could render its positioning inaccurate, and external disturbances such as thermal instability or vibrations could render it unrepeatable. High resolution enables high precision, which in turn enables high accuracy after calibration. This makes resolution vs. accuracy vs. precision an essential concept for engineers to master.

Ready to dive deeper into the world of precision stages?