No measurement is perfectly exact. Every measurement differs from the true value of the quantity being measured by some amount. This difference is known as measurement error.
In everyday language, the word “error” often implies that a mistake has been made. In metrology, however, measurement error does not necessarily indicate carelessness or improper technique. It simply reflects the fact that all measurements contain some degree of uncertainty.
Understanding measurement error is essential in science, engineering, manufacturing, and quality control because it helps determine how much confidence can be placed in a measurement result.
What Is Measurement Error?
Measurement error is the difference between a measured value and the true value of the quantity being measured.
Measurement Error = Measured Value − True Value
For example, suppose a gauge block has a certified length of 25.000 mm. If a measurement instrument reports a value of 25.015 mm, the measurement error is:
25.015 mm − 25.000 mm = +0.015 mm
In practice, the true value is often unknown. For this reason, measurements are frequently compared to accepted standards or reference values rather than a perfectly known true value.
Sources of Measurement Error
Measurement error can arise from many sources. Some originate in the measuring instrument, while others result from environmental conditions, measurement procedures, or the operator.
Common sources of measurement error include:
- Instrument calibration errors
- Worn or damaged measuring equipment
- Improper measurement technique
- Temperature effects
- Dirt, oil, or contamination
- Instrument resolution limitations
- Recording or transcription mistakes
For example, a steel part measured at a temperature significantly different from the reference temperature may expand or contract enough to affect the measurement result.
Systematic and Random Error
Measurement errors are often classified as either systematic or random.
Systematic Error
Systematic errors produce a consistent bias in the measurement result. Measurements may be consistently higher or lower than the true value.
Common causes include:
- Improper calibration
- Instrument wear
- Incorrect measurement procedures
- Environmental influences that remain constant
Because systematic errors are repeatable, they can sometimes be identified and corrected.
Random Error
Random errors cause measurements to vary unpredictably from one reading to the next.
Common causes include:
- Operator variation
- Small environmental fluctuations
- Instrument noise
- Variations in the measurement process
Random errors cannot be completely eliminated, but their effects can often be reduced through improved measurement techniques and repeated measurements.
Measurement Error and Accuracy
Measurement error is closely related to accuracy.
A measurement with a small error is generally considered more accurate than one with a large error. Reducing measurement error is one of the primary goals of calibration, quality control, and metrology programs.
However, even highly accurate measurements contain some amount of error. The objective is not to eliminate error entirely, but to understand and control it.
Controlling Measurement Error
Several practices help reduce measurement error and improve measurement reliability.
These include:
- Using properly calibrated instruments
- Following established measurement procedures
- Maintaining clean measuring surfaces
- Controlling environmental conditions
- Selecting instruments appropriate for the required tolerance
- Training personnel in proper measurement techniques
In industrial settings, measurement systems are often evaluated to determine whether they are capable of producing reliable results. Calibration programs, traceability systems, and measurement system analyses help identify and control sources of error.
Measurement Error in Manufacturing
Measurement error is particularly important in manufacturing because inspection decisions are based on measurement results.
If measurement error is large compared to the tolerance of a part, good parts may be rejected or defective parts may be accepted. For this reason, manufacturers carefully select, maintain, and calibrate measuring equipment used for inspection and quality control.
Modern quality systems place significant emphasis on understanding the capabilities and limitations of measurement systems.
Summary
Measurement error is the difference between a measured value and the true value of the quantity being measured. Because no measurement is perfectly exact, some amount of error is always present. Measurement error can result from instruments, environmental conditions, measurement procedures, or operator technique. By understanding and controlling these sources of error, engineers and technicians can improve the reliability and usefulness of measurement results.