3 Sigma Rule

     Stastically speaking:  Anytime there is a spectrum of values, there is obviously a mean average.  If you want to talk about  whether the distribution making up the average is normal, as in a bell curve or skewed toward one of the two extreme values, you can talk about by referring to a "standard deviation"  Simple stated a SD is about 16% of the spread and as such there are about six SD in a spectrum. 

     Apparently for reasons that I don't want to know, it is true that the 68-95-99 rule applies. namely that in most distributions about 68% of the values are within 16% of the average and 95% of the values are within 2 SD or 64% of the average, and of course essentially all values are within 3 SD, or 99%. 

     As far as I know this is only useful to know because so many useful things about the world are communicated by smart assholes who want to refer to the magnitude of variances as being/no being within one SD.  Hope I remember.