Garmin Xero - update

[C-130 Dude]

The Garmin is calculating SD way too low, ie optimistic. 9.3 vs 12.0. I believe the actual displayed velocities are accurate, and don't know if my friend's early-made Garmin is calculating SD wrong from three velocities, or if all Garmin's are. I'm hoping other folks with both Garmin chrons and Excel (or other apps to calculate sample SD's) can verify the results agree or not

In my short time using the Garmin I've had 2 firmware updates. The first when I set it up and the second last Saturday. Don't actually know if the update affects the calculations you mentioned.

The numbers you stated, (9.3 & 12), are these SD from a Garmin and LR? If so, who is to say which is actually correct? The comparison between the two isn't valid. The only way to accurately measure accuracy is to measure each against an industry standard chronograph. Only this measurement will provide a usable comparison. If you don't, all your getting is data output that has no basis in accuracy. That can only come from testing to a standard, not against a comparison of chrono to chrono out there. If folks continue to test one make against the other, the only result you get will be a disparity between the two chronos and BOTH may be wrong. Or maybe one will be right. It's a coin toss.

 

[Aloeus]

Ok. Here you go. For a point of reference, look at Load #2. Take the three velocities 2632.1, 2609.9 and 2628.9. Excel STDEV says 12.0, and Excel STDEVP comes up with 9.8. Garmin has 9.3. I'd say that Garmin calculates SD wrong. This is from the session with my buddy, and he got one of the early Garmin units. Perhaps it has been fixed by now. I'd be interested if any of you have three shot strings that give you the correct SD; or not. When I entered in the values from my LabRadar, my Excel SD results were EXACTLY the same results as the LabRadar.

View attachment 521732

I'm not sure where the difference between 9.8 and 9.3 is coming from. The only thing I can think of is the garmin treats decimals behind the scenes differently. I pulled a recent 5rd string and got essentially the same stdev (red is the excel calculated value)

Edit: I am also seriously impressed at GRT's ability to predict your velocities, it is never that accurate for me.

 

[C-130 Dude]

I'm not sure where the difference between 9.8 and 9.3 is coming from. The only thing I can think of is the garmin treats decimals behind the scenes differently. I pulled a recent 5rd string and got essentially the same stdev (red is the excel calculated value)
View attachment 521782

Edit: I am also seriously impressed at GRT's ability to predict your velocities, it is never that accurate for me.

Is it possible that Garmin drops anything after the period (.x)? I've not tried to calculate this, I'm only taking a jab at it...

 

[Grizzman47]

Thanks for the review....you've convinced me that it's worth picking one up.

I used my old CED M2 a few weeks ago, and didn't even mess with connecting it to my laptop to transfer the data...just manually wrote down the velocities and did the calculations myself.

 

[gregmark24]

Ok. We're there! My buddy mis-typed his Garmin-read SD on the 2nd string of three shots. He typed 9.3, and it should have been 9.8. Standard Deviation for his sample (small size) should have been 12.0, but the SD for a population (large sample size) is 9.8. Garmin calculated for the wrong type of standard deviation. From Aloeus' message above, Garmin is [still] doing it wrong. 3 shots, 5 shots is a SMALL sample size and SD should be calculated with a denominator of sample size -1, or (N-1). "Population" is a LARGE sample size with more confidence of results and is calculated with a denominator of just sample size (N). I believe Garmin is making SD look better than it is. Significantly.

 

[Aloeus]

Ok. We're there! My buddy mis-typed his Garmin-read SD on the 2nd string of three shots. He typed 9.3, and it should have been 9.8. Standard Deviation for his sample (small size) should have been 12.0, but the SD for a population (large sample size) is 9.8. Garmin calculated for the wrong type of standard deviation. From Aloeus' message above, Garmin is [still] doing it wrong. 3 shots, 5 shots is a SMALL sample size and SD should be calculated with a denominator of sample size -1, or (N-1). "Population" is a LARGE sample size with more confidence of results and is calculated with a denominator of just sample size (N). I believe Garmin is making SD look better than it is. Significantly.
View attachment 521796

I'm not going to attribute it to malice or deception- I'm not sure Garmin has anything to gain by making SDs smaller, but I do agree it really should be sample stdev. Hopefully this is something they can patch into it (or at least the option to change it).

 

[capod28]

I'm happy for those that have the Garmin. I wish I had waited a little while when I upgraded from a MS to the LR. Such is life in the ever evolving world of electronics. One month after getting a LR the Garmin was introduced.

As long as the LR is working for you, don't fret over not having the latest/greatest.

 

[JFCurl]

Below is a large sample size of 86 shots. STDEV and STDEV.P are very close with a large sample size with Excel rounded to one decimal place like the Garmin results. Originally posted with Excel having 2 decimal places which didn't really make sense since the results were being compared with the 1 decimal place Garmin.

Garmin AVERAGE	1078.8​		1078.8​	Excel Average
Garmin STD DEV	9.8​		9.9​	Excel STDEV
Garmin SPREAD	50.9​		9.8​	Excel STDEV.P
Session Note	Volq Clean Rifle Norma Tac-22	86 Shots

 

[gregmark24]

It is possible that Garmin truncates the extra digits to the right of that 0.1 place instead of rounding. Regarding your large sample vs population Excel differences, sample (STDEV) will always produce a larger result than population (STDEVP) because the denominator will be N-1 for sample and N for population, producing huge differences for small samples and minimal differences for large populations. I think the cutoff between sample and population is a size of around 30

 

[PutInOut]

Off topic; but Gregmark, it appears (in your avatar) you're sporting a Win Mod 12; my all time favorite shotgun.

 

[gregmark24]

Yes! Also my favorite. I have both the 12 and 20 ga in a 28" modified. Both old, and I typically use the 20 ga. Love them....

 

[PutInOut]

I have 3 of the 12's and one 20; hard to beat a proven winner.

 

[JFCurl]

It is possible that Garmin truncates the extra digits to the right of that 0.1 place instead of rounding. Regarding your large sample vs population Excel differences, sample (STDEV) will always produce a larger result than population (STDEVP) because the denominator will be N-1 for sample and N for population, producing huge differences for small samples and minimal differences for large populations. I think the cutoff between sample and population is a size of around 30

So if I have a case of Tac-22 (5,000 rounds). That would be the population? And if I were shooting all 5,000 rounds you would use STDEV.P. In this case, I selected two boxes to shoot so that was a sample of the 5,000 even though its a fairly large sample from a shooting stand point so the correct use would be STDEV. Another way of looking at it with much smaller numbers would be that if I reloaded 200 rounds and randomly selected 10 to test, you would use STDEV. If the following weekend I went to a competition and shot the remaining 190 rounds and added in the data from the 10 test rounds, the proper formula would be STDEV.P since it was the entire batch or population?

 

[blvette75]

When I last talked to the Garmin rep... she said they had over 24,000 back-orders for these units.

Pretty soon, guys will be able to pick these things up for $400 everywhere. lol

When these back orders hit, all the mom-and-pop shops that got conned into their 10-unit opening order, will be cutting each others throat to sell them for $5 over cost.



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Just glad I got mine been holding off buying a Labradar, just glad I waited for the new technology.