       Re: Student's T Confidence Interval [StudentTCI]

• To: mathgroup at smc.vnet.net
• Subject: [mg111351] Re: Student's T Confidence Interval [StudentTCI]
• From: Ray Koopman <koopman at sfu.ca>
• Date: Wed, 28 Jul 2010 07:25:16 -0400 (EDT)
• References: <i2ok8k\$827\$1@smc.vnet.net>

```On Jul 27, 11:55 pm, davef <davidfrick2... at yahoo.com> wrote:
> 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?

StudentTCI[ 12, 21/Sqrt, 9, ConfidenceLevel->.95 ]

{-3.0225,27.0225}

```

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