2010/10/20 David A. <dasolexa@hotmail.com>

>  I am trying to calculate confidence intervals using ci.numeric from
> epicalc package. If I generate a normal set of data and find the 99% and 95%
> CI, they seem too narrow to me. Am I doing something wrong??


Yes.
set.seed(123)
x<- rnorm(200,0,1)
ci.numeric(x=mean(x),n=length(x),sds=sd(x),alpha=0.05)
#  n                mean            sd              se  lower95ci
 upper95ci
#  200 -0.008570445 0.9431599 0.06669147 -0.1400831 0.1229422
#  Let's compute lower CI of a mean usnig t-distribution as it is in
ci.numeric
 mean(x) - qt(p = (1 - .05/2),df=length(x)-1) * sd(x)/sqrt(length(x))
# [1] -0.1400831
# now using normal distribution
 mean(x) - qnorm(p = (1 - .05/2)) * sd(x)/sqrt(length(x))
# [1] -0.1392833




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