Standard Deviation for dummies

[Bat Rastard]

Is standard deviation the number which you can statiscally expect your velocity to vary from average?
Would that be plus and minus from average?

 

[memilanuk]

If you have, for example an SD of 5... then you can generally expect your ES to range somewhere between +/-2x your SD, 95% of the time.

So 19 times out of 20, your ES will be (approximately) *4x* your SD. Mathematically speaking, of course.

The fewer shots fired - like 3 to 5 - the less consistent that will actually be.

The way I like to look at it... if you look at the numbers that your chrono spits out for say, a 10 or 20 shot string: if the ES is about 4-ish times your SD, it's probably a pretty reasonable estimate.

 

[BoydAllen]

The reason that shooters refer to SD is that chronographs calculate it. Most do not understand what it is, or how it is calculated. IMO they all need to remember that the target sees ES and even that may not matter too much shooting at shorter distances.

 

[rdb8847]

I did a Six Sigma gig at the Boeing Co. years ago and learned a ton about SD and ES. It definitely helps you understand how stable your process are.

In my opinion Velocity SD and ES only tell me how consistent my ignition (primer) and pressure (powder charge) (neck tension) are using a given lot of brass and bullets. Also assuming the barrel is constant.

I’d like to see some data plotted on target group size based on different variables.

Maybe the Shotmarker has that capability?

 

[CharlieNC]

You can easily learn all you wish by watching a few YouTube videos.