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OT-Precison vs acccuracy

CalG

Diamond
Joined
Dec 30, 2008
Location
Vt USA
I once produced several hundred Gyro caps with a case bore diameter PRECISELY .001 oversize on a spec tolerance of -.0000, +.0002" . The fact that I read the bore mic incorrectly didn't make them taste any better. The nearest reference I had was .010" above the nominal size.

If a system has precision and repeatability, Accuracy is as simple as off sets. (elevation and windage ;-)

To use the target analogy,

Regardless of the proven ability of the barrel , Random loads or variable winds will upset precision (distribution)
The sight settings will influence accuracy.
The shooter is going to influence repeatability
 

eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
An example of high accuracy with low precision would be a run of parts with a tolerance band of +/-.015" - the parts can have a wide variance in size (precision) but still be accurately sized within the tolerance band. This is kind of a matter of degree, but I absolutely get where you guys are coming from.
 

Rickyb

Cast Iron
Joined
Jan 21, 2011
Location
Troy mi
Crossthread, your reply may have been more “precise and accurate” had you mentioned to your wife the thermometers were all made in china so they are neither precise nor accurate. But no, you had to imply she was wrong. Tactical error!
 

Joe Gwinn

Stainless
Joined
Nov 22, 2009
Location
Boston, MA area
Generally you need good accuracy and ​precision if you want to do good work.

Yeahbut, what about the official definition of a meter, when it was two scratches on a long polished metal bar? Its accuracy was 100%, by definition.

So what's the precision? Basically, the width of the scratches. But but but - what about measuring things less than a meter long, and the like? Maybe 100 subdivisions - we'll call it a centimeter. So, the precision is one centimeter?

Uhh, no. That's the resolution then. Huh? Everybody calls it precision.

Lost down rabbit hole.

So what to do? All these terms have been used, generally loosely, and their definitions have become blurred, even if Marketing wasn't involved.

Anyway, the definitions I prefer are that precision is basically the number of significant decimal digits, and accuracy is how close the measurement comes to what it claims to be. Accuracy is always relative to some standard, which can be local and not official. Or it could be the international standard meter (which is no longer defined by a scratched metal bar).

 

EPAIII

Diamond
Joined
Nov 23, 2003
Location
Beaumont, TX, USA
Well, speaking about "resolution" I have to observe that your image is of poor resolution. Therefore it can not be read and it makes no point other than higher resolution would be a good thing.



I dislike the word "precision" because it is used so loosely. I was raised using 3 words: repeatability, resolution and accuracy. I made a chart similar to eKretz's in an attempt to teach high school students. The context was a discussion of the Phalanx anti-missile gun on the USS Stark, and its failure against the Iraqi exocet missile, circa 1987 (so many things to learn: Saddam was our friend, France makes impressive weapons, stuff doesn't work if it isn't turned on). Here are some of my slides. Summary is: precision = resolution.
View attachment 339444
 

CalG

Diamond
Joined
Dec 30, 2008
Location
Vt USA
The way I remember it...

Accuracy is how close you can get to the desired size, and...
Precision is how often you can do it. :D

Dennis


the way you remember these qualities is flawed. Popular, but flawed.

Precision= group size
Accuracy= how near to the bull are the groups
Repeatability =can you get the same results across the entire sheet of bulls

Is a jig borer more accurate than a bridgeport? Or is it more precise?
Is a digital display that reads out to 4 places right of the decimal more "accurate" than a handwheel dial, or is it more precise?
Is a basket of thermometers that display values "no two alike". inaccurate? of imprecise.

A cock that is broken, is Accurate twice per day!

Accurate takes a target. Precision does not. I can be precisely off the target. (intentionally)
What is improved via a "clean up pass"? Accuracy or precision. And can it be repeated? How?

Think on these things!


I'm not understanding why "common misunderstanding" should be so prevalent in this group of professionals.
 

mhajicek

Titanium
Joined
May 11, 2017
Location
Minneapolis, MN, USA
My Haas has a resolution of .001mm. Yes, it will move about two and a half clicks per tenth on an indicator, back and forth, with no perceptible backlash under zero load (though I can push the table a few tenths by hand.) When I throw in a tool and cut, with no offsets, it will cut precisely and repeatably the same amount oversize, within about a tenth, until I dial it in. Adjust the cutter offset to the midpoint of tolerance, and then it will cut accurately and repeatably until the tool begins to wear.
 

barbter

Diamond
Joined
Oct 27, 2007
Location
On Tour...
the way you remember these qualities is flawed. Popular, but flawed.
SNIP
SMIP

I'm not understanding why "common misunderstanding" should be so prevalent in this group of professionals.

This from Dennis

Accuracy is how close you can get to the desired size, and...
Precision is how often you can do it.


Was my take too.
And I guess lots of others in the UK, hence me saying the popularity of having "Precision" within the name of Engineering companies.
Also plenty of web sites (Yell.com etc) with headings of "Precision Engineers"....so perhaps in the UK at least, the term "Precision" goes hand in hand with "Accuracy"???
 

TDegenhart

Diamond
Joined
Mar 26, 2011
Location
Geneva Illinois USA
I would try to explain it this way to someone who isn't in our line of work:
Precision - to make a part exactly to size one time.
Accuracy - the ability to repeat that size on a part over and over and over.

Actually you have it backward. Accuracy is to meet a standard value. Precision is measured by the statistical variation from that standard.

Tom
 

SeeFair

Plastic
Joined
Oct 20, 2015
Well, speaking about "resolution" I have to observe that your image is of poor resolution. Therefore it can not be read and it makes no point other than higher resolution would be a good thing.

Fair comment. So here's my definitions, with regard to measurement:
Resolution: the smallest resolvable change using the device. For a ruler with 10 tick marks per inch, the resolution for the average person is 0.01", or one tenth of a tick mark (estimating between ticks).
Repeatability: the tolerance band for repeated measurements of the same object. For the example of the ruler, it might be 0.02" (do you always estimate the same?). For several people using the same device, the repeatability number maybe gets larger.
Accuracy: how close the measurement is to the "true" value. Not as good for a harbor freight scale as for a starrett, even worse for the stretchy suspenders with the inch marks on them.

I used laser feedback on a machine in open air a long time ago. Superb resolution, but repeatability and accuracy not so much, because it measured lots of things besides distance (air pressure, air temperature, air composition, etc. anything that changed air index of refraction). Putting shrouds on the beam path helped a lot with repeatability, and temperature compensation helped with accuracy. For an accurate, repeatable laser, it needs to run in a vacuum bellows (unless a lot has changed since then).
 

Joe Gwinn

Stainless
Joined
Nov 22, 2009
Location
Boston, MA area
Mostly (not always) a gaussian distribution. This is not well-understood by those who specify unilateral tolerances.

Uniform distributions are also common in practice.

The weirdest distribution commonly seen in practice is Gaussian or uniform, but with the central ~tenth of the range punched out.

This most often happens when there is a tighter-tolerance grade sold for more money - it's done by cherry-picking the run of production.

A common pattern is that the run of production is Gaussian, and what is sold as the lower-precision grade is Gaussian with central region missing, and the higher-precision grade has a uniform distribution the same width as the hole in the Gaussian.

If there are more than two precision grades, uniform-with-hole distributions will appear.
 








 
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