noone should be put through the hell of calculating sd by hand if they don't need to. I vaguely remember my stats class 10 years ago and have never had to calculate these longhand since because computer programs do it for me. let the chrony do the work.<img src=icon_smile_big.gif border=0 align=middle>

Alcohol, Tobacco, and Firearms... who's bringing the chips?

Shoot yea, don't know anyone that does it by hand. As an Engineer, I use std dev. everyday, but let excel do it for me. If you have a list of velocities or whatever, just type them in excel and there is a formula that will do it for you. You can type =stddev(a?:a?) and it will figure it for you.

Is the average and the mean the same thing? I made a c in statistics. I thought mean was the most frequently occurring? Why do chronographs even show the sd anyway? nobody knows what it means. When shooters measure groups we always report extreme spreads, never sd's for group sizes. Wouldnt sd be a better description of group size as it relates to accuracy and extreme spread more telling of velocity stats?

Not totally exactly. Reason I say this, is if you look up mean in a dictionary, it will have a synonym of average, but in some of my old books, its synonym is median which is the middle of a bunch of numbers. So if you have 100 numbers, number 50.5 would be your median but you average might be totally different.

Std. Dev. is important and does help in development in the engineering world but is only useful in shooting if you have alot of samples. I know alot of people go by extreme spread, and that works, but std. dev. kinda ignores what they call outliars or that occasional flyer.

So you would say &quot;expect velocitys of 3400fps +/- 100fps&quot;, even though you had 3 shots out of a 100 that got velocities of 3100 fps.

Handloader1, forget standard deviation.....and look at groups, if the groups are good , to hell with the standard deviation, yes groups are associated with SD.....but the SD is not what puts chucks in the ground.

Most of the posters are absolutely correct about the SD not being relevant to diameter.....it's simply a factor of variation and the number of shots fired. Let the Chrony do it for you.....then forget it.....look at the groups....screw the math.....go with the results you can see!!!

Ok, Standard Deviation and Mean Average Deviation are two distinctly different calculations. MAD is the average of shot to shot deviation compared to the overall average velocity. SD is a median average of deviation compared to the median average velocity. The calculation for each requires different steps to run the numbers used and the calculations are complex and recursive. In short - let the computer do it.

PaulS

Think safety, act safely, live long enough to enjoy it

Im think people place far too much emphasis on standard deviation.I find that the loads that produce the best groups for me seldom have the lowest standard deviation.I judge loads on group size and no longer care about standard deviation.

Vapodog,
You have the right idea. I do use MAD to help determine the best powder for a load / weapon by finding the powder that produces the lowest MAD with the smallest groups. I will work up loads with several powders and the winner is the one that does both good groups AND low MAD. That way the barrel harmonics are in conjunction with the lowest deviations in velocity and I get the best chance of consistancy in my loads.

PaulS

Think safety, act safely, live long enough to enjoy it

Guys, I guess this is the math guy talking here. But what is wrong with knowing the facts? Everytime I see a guy that doesn't grasp a concept with math or anything, they downplay it, and say &quot;screw it, you don't need it&quot;. There is nothing wrong with being a little more smarter. I agree that with shooting, usually the sample size isn't large enough to worry about SD, but handloader1 asked a good question, that he didn't know. Getting on here, basically saying &quot;don't worry about it&quot;, doesn't help him and is pretty negitive. For people like myself, it is an important number for manufacturing processes, and yea, for velocities. I guess when you get an intuitive feel for math, its like another data point worth noting.

Bigcountry:

Thanks Big Country I really enjoy mathematics. In the summer I will be going to New Mexico Tech. to study Metallurgical Engineering; I just received my Associates of Pre-Engineering Degree. After I obtain my Bachelors Degree, I hope to design bullets for somebody. Good luck.