Accuracy vs. Precision


Accuracy:
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.
More on Precision/Repeatability:
Good repeatability of a measurement (observations vary only a little
from the previous one) may indicate a precise instrument or measurement
technique but not necessarily an accurate one. In fact, an instrument
with a high resolution (the smallest discernible subdivision of an
instrument) could have poor repeatability indicating that the
instrument is not very precise and that the resolution is in fact
misleading. Similarly, a precise instrument may have a high resolution
but this should never be confused with it being accurate.
Example:
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Reply to
BottleBob
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Well done
"I am for doing good to the poor, but I differ in opinion of the means. I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it. In my youth I travelled much, and I observed in different countries, that the more public provisions were made for the poor the less they provided for themselves, and of course became poorer. And, on the contrary, the less was done for them, the more they did for themselves, and became richer." -- Benjamin Franklin, /The Encouragement of Idleness/, 1766
Reply to
Gunner Asch
BottleBob,
Agreed; the shooting targets are an excellent explanation.
But, was there a question somewhere :-)).
Wolfgang
Reply to
wfhabicher
An atta-boy from Gunner who virtually never replies to my posts? I'm duly impressed. Thanks.
Reply to
BottleBob
Wolfgang:
Consider it a public service announcement.
No question anywhere, but from a couple of posts in recent weeks where the terms were used interchangeably I thought it might be helpful to go over the basics. Sorry for boring those who already know this stuff backwards and forwards.
Reply to
BottleBob
It's worth while repeating it every now and then but it probably won't do any good. It's like explaining the difference between "there" and "their" or "site" and "sight" the folks that post on newsgroups and bulletinboards don't bother with such mundane things. Their response is nearly always "You know (sometimes no) what I mean". :-) ...lew...
Reply to
Lew Hartswick
Lew:
I know what you mean, "they're", "their", "there", plus "cite", "site", and "sight" can get confused. But realistically speaking, we're metal workers not English majors so the occasional grammatical gaffe is to be expected. The pressing question of the moment is; should the period come after or before the end quotation mark? LOL
Reply to
BottleBob
That would depend on whether you're quoting a complete sentence or not ... I'd like to be able to cite a site within your sight to clarify , but they're not about to let their info go there ...
Reply to
Terry Coombs
now explain the term "dead nuts" please? lol
Reply to
vinny
Terry:
Pretty cool. I bet you're the life of the party at all those grammar conventions.
From my Merriam Webster's "Guide to Punctuation and Style" 1995 edition on page 48, "A period is place within quotation marks even when it does not punctuate the quoted material."
Personally, I sometimes put the period on the outside, sometimes on the inside, kind of randomly. But after reading the "Guide" I better straighten up and fly right - and put them on the inside from now on. Given all the other grammatical boo-boos I'm surely making, an accurate or precise period placement is probably a minor offense.
Reply to
BottleBob
Vinny:
Impotence?
"Dead nuts" would be a colloquial term meaning as accurate AND precise as could possibly be, given the measurement system(s) in use, eh?
Reply to
BottleBob
Yours pretty much Vinny.
Reply to
John R. Carroll
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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Reply to
William Noble
And NIST.
Reply to
John R. Carroll
I agree with William. Now if you substitue the word "resolution" for "precision" I like Bob's definitions.
CarlBoyd
Reply to
Carl Boyd
I couldn't resist , as misusing words is also one of my pet peeves . To , too , two also comes to mind , as does cede, seed . And outta instead of oughta really grates on the one great nerve I have left ! I did not know that the period always goes inside ... might have at some point , but not being a wordsmith (cabinetmaker , hobby machinist) I tend to forget things like that .
Reply to
Terry Coombs
Bob, I was taught that the first word inside the quotation marks is capitalized, what does Merriam Webster say about that?
Best, Steve
Reply to
Garlicdude
William:
Well let's take a look at what some other sources have to say.
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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: ========================================================================
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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." ========================================================================
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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. ========================================================================
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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.
Reply to
BottleBob
Steve:
Here's what it says:
3. The first word of a direct quotation is capitalized. However, if the quotation is interrupted in mid-sentence, the second part does not begin with a capital.
Example: The President said, "We have rejected this report entirely."
"We have rejected this report entirely," the President said, "and we will not comment on it further."
4. When a quotation, whether a sentence fragment or a complete sentence, is syntactically dependent on the sentence in which it occurs, the quotation does not begin with a capital.
Example: The President made it clear that "there is no room for compromise."
Geeze, no more questions that entail transcription, PLEASE!
Reply to
BottleBob

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