Pumpkinheaver
Member
How do you determine standard deviation from chronograph results? Mine does not figure this for me?
Standard deviation?
- Thread starter Pumpkinheaver
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The easiest method is to use a spreadsheet program with a built in std dev function. For example, in MS Excel you would tabulate the chrono'ed velocities in a column and do a "STDEV" on that column.
The hard way is to use a calculator and do it yourself. It's not a simple formula, but it's doable.
Sum the squares of the velocities and multiply by the number of entries (n) = A
Sum the velocities and square the result = B
Subtract B from A
Divide by n * (n-1)
Take the square root. The result is the std dev.
The hard way is to use a calculator and do it yourself. It's not a simple formula, but it's doable.
Sum the squares of the velocities and multiply by the number of entries (n) = A
Sum the velocities and square the result = B
Subtract B from A
Divide by n * (n-1)
Take the square root. The result is the std dev.
dodgestdshift
Member
My scientific calculator has it already programmed. You only enter the observations, and it will calculate mean and standard deviation. It is a Texas Instruments model, but I suspect most scientific calculators do this also.
labgrade
Member In Memoriam
Good advice above, but remember too that there's the "damned lies & statistics." 
SD'll give a good indication of velocity means/yada, but the real prrof of the pudding is how they loadings actually shoot.
SD'll give a good indication of velocity means/yada, but the real prrof of the pudding is how they loadings actually shoot.
MoNsTeR
Member
And remember that the standard deviation has a sampling distribution just like the mean. That is, what you measured from a 10-shot sample is only an approximation of the true value. If the measurements are really important (eg: for competition loads) you should measure at least 30 rounds.
Worst case, you can forget calculating SD and just look at the raw data. Are the velocity listings pretty consistent? That's a small SD. Are they varying quite a bit? That's a large SD.
Not very sicentific, but that what SD is measuing--consistency of the individual pieces of data.
If you don't need the SD number for comparison between batches, I tend to agree with Monster. What matters is how they shoot.
But if you're a bench rester, or a scientific reloader, you have the options listed above.
Not very sicentific, but that what SD is measuing--consistency of the individual pieces of data.
If you don't need the SD number for comparison between batches, I tend to agree with Monster. What matters is how they shoot.
But if you're a bench rester, or a scientific reloader, you have the options listed above.
Pumpkinheaver
Member
Thanks, I have a TI calculator around here somewhere.
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