Student's T Confidence Interval [StudentTCI]

*To*: mathgroup at smc.vnet.net*Subject*: [mg111347] Student's T Confidence Interval [StudentTCI]*From*: davef <davidfrick2003 at yahoo.com>*Date*: Wed, 28 Jul 2010 02:55:13 -0400 (EDT)

I don't understqnd how this function is working. I have a sampe mean of 12, standard deviation of 21, sample size of 10 and level of significance of .05. The t critical value I get from a table for a 2-tail test (p=. 05/2=0.025 and df=10-1=9) is 2.262 My standard error of the estimate is std dev divided by square root of n = 21/(10)^1/2 = 0.0664. So my confidence interval should be 12 +/- 2.262(0.0664) = -3.0215 to 27.0215 But in Mathematica this...: \[Mu] = 12 \[Sigma] = 21 df = 10 - 1 StudentTCI[\[Mu], \[Sigma] , df, ConfidenceLevel -> .975] ...gives me this... {-44.3852, 68.3852} ...and this... StudentTCI[\[Mu], \[Sigma] , df, ConfidenceLevel -> .95] ...gives me this... {-35.5053, 59.5053} What don't I undersatnd about how to use the StudentTCI function?