标题: 99% Confidence interval and standard deviation [打印本页] 作者: domani 时间: 2011-7-13 16:44 标题: 99% Confidence interval and standard deviation
So there a question on the sample exam in the answer which they state that 99% of observations under a normal distribution will be plus/minus three standard deviations. Is this correct? My understanding was that 99% of observations under a normal distribution will be plus/minus 2.58 standard deviations作者: sgupta0827 时间: 2011-7-13 16:44
Yes, I remember seeing them both in the text at different locations. (i.e. 3 and 2.58)
I guess both are correct, 3 being more conservative than 2.58. So, if there is a choice between 3 and 2.58, I would choose 3. i.e. if a value lies between +- 2.58 standard deviations, it must for sure lie between +-3 standard deviations too.
Someone, please comment on this.作者: cchang 时间: 2011-7-13 16:44
68% values lie in between +- 1 sd
95% of values lie in between +- 2 sd
99% of values lie in between +-3sd
Never heard of using +-2.58sd :S.作者: ramdabom 时间: 2011-7-13 16:44
Yes 3 is the approximate value and the CFAI book mentions that. But if there is a choice with 3 and 2.58, i would go with the later.作者: amqata 时间: 2011-7-13 16:44
General practice is to say that +-3s. But, 2.58 is technically more correct. It doesn't matter you can use anyone you want. I would go for +-3s
