January 31, 2013


Accuracy and precision: In the fields of science, engineering, industry and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity’s actual (true) value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which repeated measurements under unchanged conditions show the same results. Although the two words reproducibility and repeatability can be synonymous in colloquial use, they are deliberately contrasted in the context of the scientific method.

Accuracy and PrecisionA measurement system can be accurate but not precise, precise but not accurate, neither, or both. For example, if an experiment contains a systematic error, then increasing the sample size generally increases precision but does not improve accuracy. The result would be a consistent yet inaccurate string of results from the flawed experiment. Eliminating the systematic error improves accuracy but does not change precision.

A measurement system is designated valid if it is both accurate and precise. Related terms include bias (non-random or directed effects caused by a factor or factors unrelated to the independent variable) and error (random variability).

The terminology is also applied to indirect measurements—that is, values obtained by a computational procedure from observed data.

In addition to accuracy and precision, measurements may also have a measurement resolution, which is the smallest change in the underlying physical quantity that produces a response in the measurement.

In the case of full reproducibility, such as when rounding a number to a representable floating point number, the word precision has a meaning not related to reproducibility. For example, in the IEEE 754-2008 standard it means the number of bits in the significand, so it is used as a measure for the relative accuracy with which an arbitrary number can be represented.

Accuracy versus precision: the target analogy: Accuracy is the degree of veracity while in some contexts precision may mean the degree of reproducibility.

The analogy used here to explain the difference between accuracy and precision is the target comparison. In this analogy, repeated measurements are compared to arrows that are shot at a target. Accuracy describes the closeness of arrows to the bullseye at the target center. Arrows that strike closer to the bullseye are considered more accurate. The closer a system’s measurements to the accepted value, the more accurate the system is considered to be.

To continue the analogy, if a large number of arrows are shot, precision would be the size of the arrow cluster. (When only one arrow is shot, precision is the size of the cluster one would expect if this were repeated many times under the same conditions.) When all arrows are grouped tightly together, the cluster is considered precise since they all struck close to the same spot, even if not necessarily near the bullseye. The measurements are precise, though not necessarily accurate.

However, it is not possible to reliably achieve accuracy in individual measurements without precision—if the arrows are not grouped close to one another, they cannot all be close to the bullseye. (Their average position might be an accurate estimation of the bullseye, but the individual arrows are inaccurate.) See also circular error probable for application of precision to the science of ballistics.

Accuracy and Performance

Analogue output: Output quantity (current or voltage signal) of a measurement system, continuous in value or time, as a response to the measured quantity.
The output supplies a current or voltage value proportional to the distance of the object.

Bandwidth: The bandwidth is the difference between an upper and lower cut-off frequency, usually 0 Hz. In telecommunications engineering the term bandwidth is sometimes also used for the amount of data which can be transferred per unit time (see Data transfer rate).

Baud rate: This indicates the number of symbols transferred in one second. The unit for this is Baud (abbreviated to Bd) or 1/s. If a symbol corresponds to one bit, then the baud rate is equal to the bit rate. See also: Symbol rate

CE directive: Products which bear the CE conformance label conform to all the EU directives relating to this product. A company whose products bear the CE label must have ensured that all directives have been fulfilled before fitting the label.

CENELEC (CLC): European committee for electrical standards. CENELEC was founded in 1973. The company has headquarters in Brussels. Most of the standards committees of the individual European nations are members.

Digital output: A digital output supplies a signal of discrete value, but not necessarily discrete in time, whose output states are limited in the circuit logic to two values. The information is transferred coded via single bits (“high” and “low” levels). This facilitates higher data rates, less losses and greater ranges.

Linearity: The maximum deviation between an ideal straight-line characteristic and the real characteristic is termed the non-linearity or linearity. The figure is given as a percentage of the measurement range (% FSO).

ODAC®: Registered trade name of the product range for optical, outside dimension measurement and control systems from Zumbach Electronic AG.

PROFINET (Process Field Network) : is the open industrial Ethernet standard of PROFIBUS & PROFINET International (PI) for automation. PROFINET uses TCP/IP and IT standards, and is, in effect, real-time Ethernet.

PROFINET IO: PROFINET IO is designed for the fast data exchange between Ethernet-based field devices and follows the provider-consumer model.

Resolution: The resolution describes the smallest possible change of a quantity which can be reliably measured by a sensor. In practice the resolution is determined by the signal-to-noise ratio, taking into account the acquired frequency spectrum.

Repeatability: Also: reproducibility, repeat accuracy, Quantitative specification of the deviation of mutually independent measurements which are determined under the same conditions







UMAC®: Registered trade name of the product range for ultrasonic eccentricity and wall thickness measurement and control from Zumbach Electronic AG.


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