[R] Homework help: Is this how CI using t dist are constructed?

[Greg Snow]

Doing:

> t.test(x)

Will give the same CI (among other things), but it is good to do it the
long way a couple of times to make sure that you understand what the
canned approach is doing (but from now on you can use the t.test
function).

Also just as a minor terminology correction, usually alpha would be 0.05
from your example, what you have as 0.95 is the confidence level
(1-alpha).

Hope this helps,

-- 
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at intermountainmail.org
(801) 408-8111
 
 

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Zembower, Kevin
> Sent: Tuesday, October 30, 2007 2:25 PM
> To: r-help at r-project.org
> Subject: [R] Homework help: Is this how CI using t dist are 
> constructed?
> 
> I'm trying to replicate some of the examples from my textbook 
> in R (my text uses Minitab). In this problem, I'm trying to 
> construct a 95% confidence interval for these distance 
> measurements [1]:
> 
> > # Case Study 7.4.1, p. 483
> > x <- scan()
> 1:  62 52 68 23 34 45 27 42 83 56 40
> 12: 
> Read 11 items
> > alpha<-.95
> > mean(x) + qt(c((1-alpha)/2, 1-((1-alpha)/2)), 
> df=length(x)-1) * sd(x)
> / sqrt(length(x))
> [1] 36.21420 60.51307
> >
> 
> Are confidence intervals with the t distribution constructed 
> using this type of equation, or am I overlooking a more 
> concise, 'canned' approach that's already been programmed? 
> Any suggestions on simplifying this?
> 
> Thanks for all your advice and help.
> 
> -Kevin
> 
> [1] An Introduction to Mathematical Statistics and its 
> Applications, fourth ed., Larsen and Marx.
> 
> Kevin Zembower
> Internet Services Group manager
> Center for Communication Programs
> Bloomberg School of Public Health
> Johns Hopkins University
> 111 Market Place, Suite 310
> Baltimore, Maryland  21202
> 410-659-6139 
> 
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