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Rifles, Reloading, Optics, Equipment
Reloading
Garmin Xero C1 Pro
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<blockquote data-quote="gregmark24" data-source="post: 3020670" data-attributes="member: 121785"><p>I didn't find IHFarmer's post, but in another thread, we've discussed this Garmin SD issue extensively. Garmin calculates (an optimistic) SD for small sample sizes. Less than 20. For 3 shots, the SD from Garmin will always be too optimistic. For 50 shots, it will be correct. As an example, let's say your velocities are 2795, 2800 and 2805 fps. Excel's STDEVP (for a [large] population) would calculate SD to be about 4.1--this is what the Garmin we used always calculated. We'd put the raw velocities for three or five shot strings into Excel and we'd get an overly optimistic SD. What Garmin SHOULD do for small groups is use a function like Excel's STDEV. This is for small group sizes, and produces a SD of 5. When we did this test two months ago, we use my LabRadar and my friend's Garmin at the same time, and for both of our rifles. For both of our rifles and 10 groups of 3 shots each, we got similar velocities between devices, but his Garmin SD's were always better than my LabRadar's SD's. We took all of the velocities, ran them in Excel, and the Garmin used the wrong method--they use SDEVP. LabRadar and Excel do the calculations correctly, and agree for small populations. I've contacted Garmin twice about this, and never heard a word back.</p></blockquote><p></p>
[QUOTE="gregmark24, post: 3020670, member: 121785"] I didn't find IHFarmer's post, but in another thread, we've discussed this Garmin SD issue extensively. Garmin calculates (an optimistic) SD for small sample sizes. Less than 20. For 3 shots, the SD from Garmin will always be too optimistic. For 50 shots, it will be correct. As an example, let's say your velocities are 2795, 2800 and 2805 fps. Excel's STDEVP (for a [large] population) would calculate SD to be about 4.1--this is what the Garmin we used always calculated. We'd put the raw velocities for three or five shot strings into Excel and we'd get an overly optimistic SD. What Garmin SHOULD do for small groups is use a function like Excel's STDEV. This is for small group sizes, and produces a SD of 5. When we did this test two months ago, we use my LabRadar and my friend's Garmin at the same time, and for both of our rifles. For both of our rifles and 10 groups of 3 shots each, we got similar velocities between devices, but his Garmin SD's were always better than my LabRadar's SD's. We took all of the velocities, ran them in Excel, and the Garmin used the wrong method--they use SDEVP. LabRadar and Excel do the calculations correctly, and agree for small populations. I've contacted Garmin twice about this, and never heard a word back. [/QUOTE]
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