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Old July 02, 2018, 13:10   #1
gunplumber
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Reference - FAL - IMBEL Locking shoulder Size Distribution

moved to Ref & Data

So I was sorting locking shoulders and thought, it would be cool to calculate the average size and distribution of in-service IMBEL rifles. It's been almost 30 years since I took graduate statistics and math has never been my strong point.

I have around 600 that I hadn't included in kits and with my summer help back in school - they weren't going to sort themselves.

I set aside all that were "new" and counted only those I pulled from receiver sections (yeah, that was a chore!)
I discarded all damaged ones.
I discarded or set aside any that had been modified

This is my distribution entered into Excel

size Qty (fx size x qty)
0.268 12 3.216
0.267 17 4.539
0.266 20 5.320
0.265 34 9.010
0.264 74 19.536
0.263 59 15.517
0.262 91 23.842
0.261 61 15.921
0.260 69 17.940
0.259 45 11.655
0.258 45 11.610
0.257 29 7.453
0.256 38 9.728
0.255 21 5.355
0.254 15 3.810
0.253 4 1.012

Total 634
fx sum of size x qty = 165.464
fx mean (total size / total qty) 0.2610
median
mode 262 (that's easy)
range .015

Conclusions -
.261" is the average (mean) size;
.262" is the most frequently occurring size (mode)
.264" is the second most frequently occurring size
.260" is the third most frequently occurring size
.263" is an anomaly making a non-standard distribution

How do I calculate the median using excel without entering 634 separate values?
can I also calculate standard deviation without entering 600 data points?
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Old July 02, 2018, 14:41   #2
meltblown
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Your median is the average of values 316 and 317. Since .261 occurs from a low of 267 counts and a high 327. .261 is the median. On a spread sheet you could just add one box to the previous to find the values where the count lies between 316 and 317
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Old July 02, 2018, 19:28   #3
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I don’t understand your method if you want a decent approximation iof standard deviation. I would take the data as a percentage of the population and then run the STD on the resultants you got away from the normalization of the units. 1 is always the number statistics are an estimation. That data is fairly normal. 1 standard deviation is 62% 2 is 97.5 and 3 is 99.7. Divide by the average and you get the variance of which plus or minus is what you see 62% of the time
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Last edited by meltblown; July 02, 2018 at 19:41.
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