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Standard Deviation (SD or Std Dev) is how far away your actual results are from expected results -- ie: variance.
It's a way to express how likely or unlikely certain results are.
People use 3 SD's as a way to basically encompass everything that is possible. Of course, 4 SD's is also possible, and 5....but the probability of those is so low, you can for the most part, just throw those obscene chances out.
Originally Posted by
Meistro123
I think you might have it mixed up. Being within 3 SD's is 99.73% chance. So being +3 SD or more up is half of 0.27%, or about 0.135% (about 1 in 740). Being down 3 SD's is the same, 0.135% or 1 in 740.
This is an interesting chart from the wikipedia link from Meistro:
Code:
Range Expected Fraction of
Population Inside Range
Approximate Expected
Frequency Outside Range
Approximate Frequency for Daily Event
? ± 0.5? 0.382924922548026 2 in 3 Four times a week
? ± ? 0.682689492137086 1 in 3 Twice a week
? ± 1.5? 0.866385597462284 1 in 7 Weekly
? ± 2? 0.954499736103642 1 in 22 Every three weeks
? ± 2.5? 0.987580669348448 1 in 81 Quarterly
? ± 3? 0.997300203936740 1 in 370 Yearly
? ± 3.5? 0.999534741841929 1 in 2149 Every six years
? ± 4? 0.999936657516334 1 in 15787 Every 43 years (twice in a lifetime)
? ± 4.5? 0.999993204653751 1 in 147160 Every 403 years (once in the modern era)
? ± 5? 0.999999426696856 1 in 1744278 Every 4776 years (once in recorded history)
? ± 5.5? 0.999999962020875 1 in 26330254 Every 72090 years (thrice in history of modern humankind)
? ± 6? 0.999999998026825 1 in 506797346 Every 1.38 million years (twice in history of humankind)
? ± 6.5? 0.999999999919680 1 in 12450197393 Every 34 million years (twice since the extinction of dinosaurs)
? ± 7? 0.999999999997440 1 in 390682215445 Every 1.07 billion years (a quarter of Earth's history)
NOTE: Those probabilities are for being WITHIN +/- a given SD. For only being + or only being - a specific SD, cut the number or probability in half.
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