Technology
Technical Note
Zero Offset, Accuracy, Precision, Resolution
No measurement is 100% exact and no measurement instrument is 100% perfect.
When a quantity is measured, the outcome depends on the measuring system, the
measurement procedure, the skill of the operator, the environment, and other effects.
Even if the quantity were to be measured several times, in the same way and in the
same circumstances, a different measured value would likely be obtained each time,
assuming the measuring system has sufficient resolution to distinguish between the
values.1
The following terms will be described in this technical note.
• Zero Offset
• Accuracy
• Precision
• Resolution
Not Accurate
Not Precise
Zero offset
The difference between true zero and the indication given by the measuring
instrument. Some measuring instruments, due to minor manufacturing fault, do not
read zero but a small negative or positive value when it is not connected for any
measurement. That initial reading is called zero offset error. The user has to deduct
that value from all measurements.
Accuracy
Accurate:
Not Precise
The closeness of agreement between a measured value and a true or accepted value
(measurement error reveals the amount of inaccuracy).
Precision
A measure of the degree of consistency and agreement among independent
measurements of the same quantity (also the reliability or reproducibility of the
result). Think of it like you are playing darts (see examples on right). Precision is
really about detail. It has nothing to do with accuracy. Accuracy is about giving true
readings, not detailed readings.
Resolution
Characterizes the ability of instrument’s output or display to show changes in the
quantity. Resolution is formally defined as the smallest change or increment in the
measured quantity that the instrument can detect. For digital instruments, resolution
is usually associated with the number of digits displayed on the output. E.g., a
voltmeter with 5 digits has better resolution than one with 4 digits.
• Be careful – an instrument can be very precise, but not very accurate, and viceversa. Furthermore, a high-resolution instrument may be neither precise nor
accurate!
An stopwatch example to graphically distinguish resolution from precision and
accuracy follows on the next page.
Basic Mechanical Engineering, Pravin Kumar, copyright 2013 Dorlilng Kindersley (India) Pvt. Ltd. P 439.
1
DefinitionofTerms(TTU-20tn)RevA2015USltr
Not Accurate
Precise
Accurate
Precise
Technology
Zero Offset, Accuracy, Precision, Resolution cont.
Stopwatch example:
1. Poor resolution, accurate
2. Poor resolution, inaccurate
3. Excellent resolution, inaccurate
4. Excellent resolution, accuracy
The four stopwatches above are used to measure the same time span. The exact time is 55.768331 seconds.
What can be said about the accuracy, precision and resolution of each?
• The first stopwatch has poor resolution – only two digits. But it does the best job possible within its
limits. It is accurate because its reading matches the exact reading rounded to two digits.
• The second stopwatch is not very accurate - it displays an inaccurate reading compared to the exact
value, AND it has poor resolution only displaying two digits.
• The third stopwatch has excellent resolution – displaying five digits, including the thousandths place.
The reading, however is not accurate.
• The fourth stopwatch is both accurate and has good resolution.
• Nothing can be concluded about precision as each watch has only one reading.
Transonic prides itself on the accuracy of its measurements. We offer researchers, and now clinicians, highly
accurate measurements at low flows with a very high degree of resolution. As Transonic has challenged
the limits in quantifying flow with its microvascular application, these concepts are all the more critical to
understand.
REFERENCES:
Adapted from: Errors in Calibration, John M Cimbala, Pen State University, latest revision: 27 August 2009.
http://mrsmithsphysics.weebly.com/uploads/1/2/1/5/12150755/1.2_errors_and_uncertainties_notes.pd
Transonic Systems Inc. is a global manufacturer of innovative biomedical measurement
equipment. Founded in 1983, Transonic sells “gold standard” transit-time ultrasound
flowmeters and monitors for surgical, hemodialysis, pediatric critical care, perfusion,
interventional radiology and research applications. In addition, Transonic provides
pressure and pressure volume systems, laser Doppler flowmeters and telemetry systems.
www.transonic.com
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