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Re: precision/accuracy



I sent this off earlier, but didn't see it show up. Apologies to list
members if it actually shows up twice. -lois



Before this year, I used to separate precision and accuracy. I used
precision to describe the "random" error and accuracy to describe the
"systematic" error. I realized this was simplifying things, but I felt it
was sufficient for the students. Unfortunately, I can't succinctly
describe the problem I had with that approach.


I see the need to make things concrete and memorable for most of my
students, and since this is one of the first topics of the semester, I also
use it to impress my students. ;-)

I teach precision vs. accuracy with a set of 4 targets; real targets (50
yard type) with real .38 holes in them. I tell a story that I had a new
.38 cal. revolver, and needed to give it a try.

I loaded my new revolver with 6 rounds of the usual target practice type
semi-wab cutter ammo, and fired at the target. That first target has
widely spaced holes, randomly distributed. So I tell them there's
obviously something wrong. There is neither accuracy nor precision, and
I'm not THAT bad a shot! My first guess is the cheap ammo, so I go and
get the match grade ammunition.

Match grade ammunition is certified to have (within measurement
capabilities) the same mass bullet and the same amount of powder in every
round. I loaded my weapon with the match grade ammunition and fired 6
rounds at the next target. This second target I show has 6 rounds, all
clustered nicely together, in a tight grouping, but the cluster is about 4
inches from the bull's eye at 3 o'clock. I have precision but no accuracy.
Everything is the same, but it's not where it should be. But, the story
continues, it's a new revolver, and I haven't sighted it in yet.

So I reload with 6 rounds of match grade ammunition, and take my
handy-dandy little screw driver. I put the first 2 rounds in the target,
and show them at a high and away location. I figure it has to come over and
down, so I adjust the rear sight, and put two more rounds in the traget.
Better, but still high. Adjust again, this time only raising the rear
sight, and the last two rounds in the target. Voila, the last two are
dead on. I have improving accuracy, but I've lost precision overall,
because I've been changing something each time.

Now I have consistent, match grade ammunition, and the weapon is sighted
in. six more rounds at the last target, and they are tightly grouped,
right in the center. :-) Now *that's* how I usually shoot!

The kids love it, and never forget it. Not to mention I have acquired the
respect of the jocks and Greeks in the back row!

Dr. Lois Breur Krause
Department of Geological Sciences
442 Brackett Hall
Clemson University
Clemson SC 29634

krause@clemson.edu
http://home.earthlink.net/~breurkrause

If I had one wish for our nation,
I would wish for a turn about of what we value in a person.
We value athletic prowess, and not intelligence.
We value physical attractiveness, and not beauty of the soul.
We value cunning and wealth, and not honesty and integrity.
These lessons should be taught to our youth.
Instead our schools demand athletic competition of every student,
and denigrate the scholar.
Failing to reward excellence rewards failure. - Dr. L. Breur Krause