Accuracy Versus Precision

[Art Eatman]

As NOAA sees it:

http://celebrating200years.noaa.gov/magazine/tct/tct_side1.html

"A surveyor strives for both accuracy and precision. Many people use the terms “accuracy” and “precision” interchangeably. However, for those in the surveying profession (as well as other technical and scientific fields), these words have different meanings. To surveyors, “accuracy” refers to how closely a measurement or observation comes to measuring a "true value," since measurements and observations are always subject to error. “Precision” refers to how closely repeated measurements or observations come to duplicating measured or observed values.

Using four cases of rifle shots fired at a bull’s eye target, each with different results, helps to distinguish the meaning of these two terms."

 

[Casefull]

Yes. You use the term 'repeated measurements'. The more repeatable all of the functions that we, the machine, the ammo, and the weather are capable of the better the 'accuracy' or precision becomes. Seems the words are different shades of the same general idea. An accurate reading on my micrometer leads to greater precision in the assembly of the crankshaft main bearings....and so it goes.

 

[wally]

A broken watch is accurate twice a day. But its never precise. A watch that runs 1 minute slow is never accurate but can be very precise (if its a good watch )

In shooting, accuracy is the average deviation of the bullets point of impact from the point of aim. Precision is the average group size. But most people simply call precision "accuracy" because the POA/POI can be adjusted, but its really not quite that simple.

 

[Apuuli]

A ten inch group centered exactly on the center of the bullseye is accurate but not precise.

A half inch group centered 6 inches to the left of the bullseye is precise but not accurate.

A ten inch group that centered 6 inches to the left of the bullseye is neither accurate nor precise.

A half inch group centered exactly on the bullseye is both accurate and precise.

 

[CraigC]

Yep, most shooters (including myself) incorrectly use the term "accurate" but it's a tough habit to break. Although I'm doing pretty good with "caliber" vs. "cartridge" or "chambering".

 

[Smokey Joe]

Accuracy vs. precision...

Art--The use of the term "accurate" goes back a long way.

As no less a figure than the revered Col. Townsend Whelen put it, "Only accurate rifles are interesting."

And yet I hear some speak of "precision shooting" when they mean centering tiny groups on the bullseye.

I think that your observation

Many people use the terms “accuracy” and “precision” interchangeably.

is in itself an accurate one.

 

[General Geoff]

wally, I like the 'broken watch' analogy.

 

[wombat13]

A ten inch group centered exactly on the center of the bullseye is accurate but not precise.

A half inch group centered 6 inches to the left of the bullseye is precise but not accurate.

A ten inch group that centered 6 inches to the left of the bullseye is neither accurate nor precise.

A half inch group centered exactly on the bullseye is both accurate and precise.

This is correct. Precision is meaningless without accuracy and vice versa. If I miss the deer exactly three inches high with four shots, who cares? I was precise but I still missed. If I miss the deer four times, but the "group" center is on the deer's vitals, who cares? I was accurate but I still missed.

The way I see it, every shooter is trying to be accurate with an acceptable level of precision. What level of precision is acceptable depends on the context. If I'm taking a 25 yard shot at a deer with my rifle, I don't have to worry about it too much. I have to be much more careful if I'm taking a 600 yard shot.

As another poster stated, for the most part when people talk about accuracy they mean precision, probably because precision is usually the more difficult part (i.e., adjust the scope/sights to correct accuracy problems).

 

[SSN Vet]

The way I see it, every shooter is trying to be accurate with an acceptable level of precision.

Agreed,

I'm not a competitor myself, but when we scored targets for our proficiency quals long ago. tiny little groups clustered outside of the outer ring didn't contribute to a high score.

And by virtue of the fact that the target consists of concentric circles with a small bulleseye, the highest scores must be both accurate and precise.

 

[MCMXI]

Although I'm doing pretty good with "caliber" vs. "cartridge" or "chambering".

You and me both. It's laziness and/or ignorance pure and simple and firearms manufacturers are just as guilty. A CALIBER has units of inches or mm and NOTHING ELSE!! It's a DIMENSION for God's sake!! :banghead:

OK ... rant off.

I don't have much regard for benchrest shooting. It's useful in that it provides a data point (or two) for the system (rifle, optics, rests and ammunition), but any sport that rewards the smallest group regardless of where it is on the target holds no interest for me. Even golf requires more of each competitor!! F-Class, Service Rifle and Practical Shooting reward accuracy AND precision which are applicable to every real-world aspect of shooting. In F-Class, the accuracy and precision requirements are neatly wrapped up into two "simple" objectives. To score 100%, no shot can be more than 0.50 MOA away from the center of the target. To score 100%-100%X, no shot can be more than 0.25 MOA away from the center of the target. Simple right? Now take a look at the National F-Class records and see how many records have been set that meet the +/- 0.25 MOA criterion (bottom of list on left side in link below).

http://www.nrahq.org/compete/highpower.asp

 

[JohnBT]

"A half inch group centered 6 inches to the left of the bullseye is precise but not accurate."

I disagree, a small group like that is frequently shot off to the side on purpose. Groups are shot off to one side of the bullseye so as not to destroy the aiming point.

It's a cute statement you've made, but it's not precise or accurate.

 

[skiking]

"A half inch group centered 6 inches to the left of the bullseye is precise but not accurate."

I disagree, a small group like that is frequently shot off to the side on purpose. Groups are shot off to one side of the bullseye so as not to destroy the aiming point.

It's a cute statement you've made, but it's not precise or accurate.

Wrong. The accuracy is how close your group is to your point of aim. Lets say for some reason, you are building something, and for whatever reason, all of your lumber needs to be 2" shorter than what is called for on the plans and lets say you can't add. So you cut 2" off of your tape measure and measure all of your cuts to what is called for on the plans. Great, now you don't have to do any math, and all of your cuts come out perfect, but now your tape measure isn't very accurate.

By adjusting your sights so that you don't destroy your aiming point, you are degrading the accuracy, even though doing so provides a desirable outcome.

 

[Gryffydd]

Groups are shot off to one side of the bullseye so as not to destroy the aiming point.

I think it's pretty clear in this context that "bullseye" = point of aim. It's your interpretation that's not accurate.

 

[wally]

wally, I like the 'broken watch' analogy.

Can't claim it as original, it came from one of my very first engineering classes, but it does nicely illustrate the difference!

 

[JohnBT]

"It's your interpretation that's not accurate."

Nope, you're stuck on your world view and the facts don't support it. The fact that they shoot teeny tiny groups prove that they know what they are doing and putting the shots where they are intended to go.

 

[Slamfire]

I don't remember the Artillery FM that has an explanation for CEP and offset, but this URL does a very good job of describing the concept.

http://www.weaponeering.com/tech_corner.htm

 

[Art Eatman]

"...a small group like that is frequently shot off to the side on purpose."

"Frequently". That doesn't mean "always". For the purposes originally described, it looks to me like the A vs. P thing holds. As usual, situational and for explanatory purposes. Let's don't go huntin' boogers, okay?

 

[Dr T]

This poem illustrates a interesting point about the interplay between the two.

http://www.columbia.edu/~to166/hiawatha.html

Kendall, Maurice (1959). Hiawatha Designs an Experiment. The American Statistician 13: 23-24.

This was brought home to me in my first year at UTAustin. I took a surveying course in which we had to measure the length of the East Mall. It did convince me that the Gaussian (Normal) probability distribution did reasonably describe repeated measurements and that minimization of the standard deviation was a good thing.

But practically, I try to find a combination of bullet and powder that will give me reasonable 3 shot groups (preferably less than 1"), and then go hunting.

And there is a difference between a practical solution (the 90% you get with 10% of the work) and the ultimate in precision.

 

[JohnBT]

Okay, some folks seem to be insisting that a 6 o'clock pistol hold is NOT precise AND accurate because the shooter isn't aiming at the center of the bull.

I still don't agree that the POA has to equal the POI for a gun to be both accurate and precise. Any bullseye pistol shooters out there?

John

 

[JohnBT]

For the rifle-only shooters, here's what I'm referring to. Two different ways of aiming a firearm, in this case pistols. One way is POA=POI and the other POA does not equal POI - the bullets strike higher.

 

[NG VI]

A ten inch group centered exactly on the center of the bullseye is accurate but not precise.

A half inch group centered 6 inches to the left of the bullseye is precise but not accurate.

A ten inch group that centered 6 inches to the left of the bullseye is neither accurate nor precise.

A half inch group centered exactly on the bullseye is both accurate and precise.

I think your use of accurate and precise in the first two examples are flopped. Accuracy is repeatable results, precision is getting the result you want. In the ten inch centered group example, you are getting as much precision as is possible, since your shots are all on target, your accuracy just sucks because your projectiles are not converging to a more useful degree.

I think.

 

[Maverick223]

I think your use of accurate and precise in the first two examples are flopped. Accuracy is repeatable results, precision is getting the result you want.

Nope, he has it right...the shooting sports is what makes things confusing as the terms are not typically used correctly (myself included).

Here is a little more information about accuracy vs. precision.

 

[wombat13]

I think your use of accurate and precise in the first two examples are flopped. Accuracy is repeatable results, precision is getting the result you want. In the ten inch centered group example, you are getting as much precision as is possible, since your shots are all on target, your accuracy just sucks because your projectiles are not converging to a more useful degree.

I think.

No, the original post is correct. Perhaps this analogy will help.

Let's assume that it is 4:15 pm when someone walks into my office. The visitor asks what time it is. I look out the window to see the sun and say that it is late afternoon. My answer is correct but is vague, i.e., it is accurate but not very precise.

My officemate responds that I'm wrong. It is 2:01 in the afternoon he says after looking at the clock, not realizing that the clock stopped. His response is not vague at all, but he is wrong. His answer is precise but not accurate.

 

[MCMXI]

Enough with the analogies!! :banghead: This isn't complicated folks!! In fact, I find it amusing that NOAA chose to use four targets/groups (shown below) to illustrate the difference between ACCURACY and PRECISION, and then members here, who presumably are shooters, need to talk about watches and the time of day. How about a show of hands if you think the targets (as shown by NOAA) aren't sufficient to illustrate the difference?!! We can spend days talking about statistical methods, normal Gaussian distributions etc., but at the end of the day, the objective for most of us is simple enough. Shooting has always been a mental game. Many, many shooters get too comfortable with one kind of target at one range and one position. Something as simple as changing the type of target can have a dramatic effect on accuracy and precision. You may be surprised at how many shooters lose confidence when the target is only 1 MOA in size (steel plate for instance) ... particularly at 300 yards and beyond.