Accuracy vs. Precision

vinny wrote:


Yours pretty much Vinny.
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John R. Carroll
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lol
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vinny wrote:

You didn't think I was going to say mine did you? Hehe.
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John R. Carroll
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wrote:

Hairy Balls on an old, ugly, fat man, with no money.
Tom
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These are not exactly as I would describe it -
Accuracy - the degree to which the measured value matches the true value (not "expected") Precision - the resolution of measurement - number of significant digits - precision must be greater than or equal to accuracy repeatability - the degree to which successive measurements of the same exact object return the same value
so, my understanding of precision seems to differ from yours
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William Noble wrote:

And NIST.
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John R. Carroll
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I agree with William. Now if you substitue the word "resolution" for "precision" I like Bob's definitions.
CarlBoyd
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Carl Boyd wrote:

Carl:
    Ok, here is what NIST (National Institute of Standards and Technology) has to say about it, and I would tend to put more faith in their interpretation than other Usenet sources.
===================================================================http://physics.nist.gov/Pubs/guidelines/appd.1.html
D.1.1.1 accuracy of measurement [VIM 3.5] closeness of the agreement between the result of a measurement and the value of the measurand
NOTE "Accuracy" is a qualitative concept.
The term "precision" should not be used for "accuracy."
TN 1297 Comments: 1. The phrase "a true value of the measurand" (or sometimes simply "a true value"), which is used in the VIM definition of this and other terms, has been replaced here and elsewhere with the phrase "the value of the measurand." This has been done to reflect the view of the Guide, which we share, that "a true value of a measurand" is simply the value of the measurand. (See subclause D.3.5 of the Guide for further discussion.)
2. Because "accuracy" is a qualitative concept, one should not use it quantitatively, that is, associate numbers with it; numbers should be associated with measures of uncertainty instead. Thus one may write "the standard uncertainty is 2 µΩ" but not "the accuracy is 2 µΩ."
4. The VIM does not give a definition for "precision" because of the many definitions that exist for this word. For a discussion of precision, see subsection D.1.2.
D.1.1.2 repeatability (of results of measurements) [VIM 3.6] closeness of the agreement between the results of successive measurements of the same measurand carried out under the same conditions of measurement.
D.1.1.3 reproducibility (of results of measurements) [VIM 3.7] closeness of the agreement between the results of measurements of the same measurand carried out under changed conditions of measurement
D.1.2 As indicated in subsection D.1.1.1, TN 1297 comment 4, the VIM does not give a definition for the word "precision." However, ISO 3534-1 [D.2] defines precision to mean "the closeness of agreement between independent test results obtained under stipulated conditions." Further, it views the concept of precision as encompassing both repeatability and reproducibility (see subsections D.1.1.2 and D.1.1.3) since it defines repeatability as "precision under repeatability conditions," and reproducibility as "precision under reproducibility conditions." Nevertheless, precision is often taken to mean simply repeatability. ==================================================================    Note the very last sentence.
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BottleBob
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William Noble wrote:

William:
    Well let's take a look at what some other sources have to say.
=======================================================================http://www.engr.mun.ca/~cdaley/1000/measurem.htm
Precision
If you repeat a measurement several times on the same parameter over the period of measurement, you may get a series of readings that differ from each other. The cause may be small differences in how you use the instrument each time. The differences could also be due to random changes in the instrument, and they could be due to small changes in the parameter you are measuring. Whatever the cause, you would be inclined to take the average of the readings as the best value you can quote or use. You can get an idea of the variability from the range of values obtained, i.e. the difference between the largest and smallest reading, but a better measure is likely to be the variance of the readings. Variance, var, is a statistical measure obtained by calculating: =============================================================================================================================================http://honolulu.hawaii.edu/distance/sci122/SciLab/L5/accprec.html
    Accurate means "capable of providing a correct reading or measurement." In physical science it means 'correct'. A measurement is accurate if it correctly reflects the size of the thing being measured.
    Precise means "exact, as in performance, execution, or amount. "In physical science it means "repeatable, reliable, getting the same measurement each time." ======================================================================     =======================================================================http://scidiv.bcc.ctc.edu/Physics/measure&sigfigs/B-Acc-Prec-Unc.html
Accuracy
Accuracy refers to the agreement between a measurement and the true or correct value. If a clock strikes twelve when the sun is exactly overhead, the clock is said to be accurate. The measurement of the clock (twelve) and the phenomena it is meant to measure (The sun located at zenith) are in agreement. Accuracy cannot be discussed meaningfully unless the true value is known or is knowable. (Note: The true value of a measurement can never be known. Read more about this.)
Accuracy refers to the agreement of the measurement and the true value and does not tell you about the quality of the instrument. The instrument may be of high quality and still disagree with the true value. In the example above it was assumed that the purpose of the clock is to measure the location of the sun as it appears to move across the sky. However, in our system of time zones the sun is directly overhead at twelve O'clock only if you are at the center of the time zone. If you are at the eastern edge of the time zone the sun is directly overhead around 11:30, while at the western edge the sun is directly overhead at around 12:30. So at either edge the twelve O'clock reading does not agree with the phenomena of the sun being at the local zenith and we might complain that the clock is not accurate. Here the accuracy of the clock reading is affected by our system of time zones rather than by any defect of the clock.
In the case of time zones however clocks measure something slightly more abstract than the location of the sun. We define the clock at the center of the time zone to be correct if it matches the sun, we then define all the other clocks in that time zone to be correct if they match the central clock. Thus a clock at the Eastern edge of a time zone that reads 11:30 when the sun is overhead would still be accurate since it agrees with the central clock. A clock that read 12:00 would not be accurate at that time. The idea to get used to here is that accuracy only refers to the agreement between the measured value and the expected value and that this may or may not say something about the quality of the measuring instrument. A stopped clock is accurate at least once each day.
Precision
Precision refers to the repeatability of measurement. It does not require us to know the correct or true value. If each day for several years a clock reads exactly 10:17 AM when the sun is at the zenith, this clock is very precise. Since there are more than thirty million seconds in a year this device is more precise than one part in one million! That is a very fine clock indeed! You should take note here that we do not need to consider the complications of edges of time zones to decide that this is a good clock. The true meaning of noon is not important because we only care that the clock is giving a repeatable result. =============================================================================================================================================http://www.ee.unb.ca/tervo/ee2791/intro.htm
The precision of an instrument reflects the number of significant digits in a reading;
The accuracy of an instrument reflects how close the reading is to the 'true' value measured. ======================================================================    This last one above seems to support your perspective.
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BottleBob
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giant snip

well, it seems that there is no universal agreement, and further, that the meaning differs if you are refering to measurments or to something else.
you can say the time is precisely 3:00 - that has a resolution of one minute, a precision of one minute, but if the actual time is 6:35, its accuracy is only 3.5 hours - this is illustrated by the classic problem of excess decimal places.
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Accuracy refers to the closeness between measurements (observations) and their expectations ("true" values). The farther a measurement is from its expected value, the less accurate it is.
Precision:
Precision pertains to the closeness to one another of a set of repeated observations of a random variable. Thus, if such observations are closely clustered together, then these observations are considered to have been obtained with high precision.
Well, from subsequent comments, that may be good English, but to us Yobbo high school graduates, it requires too much thought to comprehend.
From my (admittedly limited) engineering trade lessons, what I was taught was
Accuracy - the ability to measure or layout to the tolerances needed for the job. Precision - the ability to be able to work to those measurements and tolerances.
Andrew VK3BFA
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wrote:

I think of it like shooting a gun. To hit the target takes accuracy. To hit the target every freaking time takes precision.
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maybe my memory is fading, but I seem to recall, back in the good old days of slide rules, that we used the word "precision" to mean "Resolutoin" as you define it - I tend to think of resolution as something lenses have, not micrometers, but I can be comfortable with it meaning "precision"
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Unfortunately, I have seen these two terms misused even in advertising for otherwise good products. I think firms for which these terms are important ought to have technical people approve their ads, but I guess that is too much to hope for in today's business world.
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wrote:

And here I thought that the target illustration in BottleBob's first post was clear and self-explanatory!
I'd used that analogy in my work measurement class and the students seemed to grasp the idea.
Silly 'ol me!
Wolfgang
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