# Accuracy vs repeatability



## BWSmith (Aug 24, 2010)

http://en.wikipedia.org/wiki/Accuracy_and_precision

Not beating a horse here.......just trying to clear what some have difficulty understanding when the topic moves twds metrology.

In my pea brain repeatabilty is the function of a fixture/jig,after its made.The accuracy or precision is more,"by design".JMO but,WWers tend to,by nature require more repeatability.......than true precision.But thats wholey dependant on the project at hand.In alot of cases in our shop accuracy is more of a requirement,since its a "one-off".Understanding the difference may free up some grey matter.BW


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## firemedic (Dec 26, 2010)

Your right, BW... except about freeing up grey matter... That all fell out yrs ago... lol

~tom ...it's better to remain silent and be thought a fool than to open one's mouth and remove all doubt...


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## BWSmith (Aug 24, 2010)

Heres one thought on grey matter........."the further one gets from the basic's,the further you'll have to travel to get back to them"

Insert,longterm vs shorterm memory right here.

The "basic's" in any biz,sport,hobby,ect...IMO are the foundation on which your education/edification is built.Make that foundation as strong as you can........preserving grey matter in the process.

Everybody loves creativity,just don't use that as an excuse.BW


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## GeorgeC (Jul 30, 2008)

BWSmith said:


> http://en.wikipedia.org/wiki/Accuracy_and_precision
> 
> Not beating a horse here.......just trying to clear what some have difficulty understanding when the topic moves twds metrology.
> 
> In my pea brain repeatabilty is the function of a fixture/jig,after its made.The accuracy or precision is more,"by design".JMO but,WWers tend to,by nature require more repeatability.......than true precision.But thats wholey dependant on the project at hand.In alot of cases in our shop accuracy is more of a requirement,since its a "one-off".Understanding the difference may free up some grey matter.BW


My grey matter is sadly lacking.

I do not understand just what it is that you are trying to discuss. What is the purpose of this post?

G


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## BWSmith (Aug 24, 2010)

George,its a fundemental in metrology.........read the link.Look at the part about arrows hitting a specific mark vs how tight a "group" you can shoot.Sometimes its more important for a measure to be gaged to a known standard........other times its more important that every pc is the same,in which "that" becomes the std.

If you're not concerned with either in your shop........well,thats OK.Many aren't.BW


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## woodnthings (Jan 24, 2009)

*try this*

Let's say you wanted 10 pieces of stock 12 1/16 " long, that being the "target". Your set up made 10 pieces 12" long all the same. You missed the target (accuracy) but they were precisely all the same.
The other case would be they varied in length between 11 15/16" and 12 1/8", neither accurate or precise. Another case is that 6 were 12 1/16" and 6 were 12 ", depending on the range of acceptability or tolerance, that may work fine. If they need to fit within a space they will all fit. If the pieces were 12 1/8" they will not fit. sometimes the designation (-1/16" + 0")

Tolerance vs allowance: http://en.wikipedia.org/wiki/Allowance_%28engineering%29#Confounding_of_the_engineering_concepts_of_allowance_and_tolerance

Terminology defined:
http://www.me.unlv.edu/Undergraduate/coursenotes/Tolerance and fit.htm

http://en.wikipedia.org/wiki/Accuracy_and_precision
With regard to the arrow link above, All the first arrows hit the target, and the bull's eye. The second arrows made a tighter group which would mean the set up is precise just needs some adjustment to bring it into specs. The "bad guy" got killed in all cases, but I want the archer or in a different case (gun, scope and sniper) in the second group on my side.


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## H. A. S. (Sep 23, 2010)

Statistical Process Control. SPC.:smile:


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