R help - Apr 2002 - simple Q: required sample size & usage of power.t.test()

Hi community,

Seems I am too stupid:

I try to calculate with power.t.test() an example from the statistics textbook
[Jerrold H. Zar, Biostatistical Analysis, 2nd edition, Prentice Hall, 1984,
Chapter 8, Exercise 8.4, (page 120)]:

-------
A sample of size 18 has a mean of 13.55cm and a variance of 6.4512cm^2.
(a) Calculate the 95% confidence interval for the population mean.
(b) How large a sample would have to be taken from this population to estimate µ
to within 1.00cm,
 with 95% confidence?
(c) 2.00cm with 95% confidence?
(d) 2.00cm with 99% confidence?

Result should be - according to the answers appendix - (a) 13.55 +- 1.26cm (b)
n=29, (c) n=10, (d) n=11
-------

Here my questions:

(1) which functions in R can be used to solve (a)
(2) what has to be the syntax to answer (b), (c) and (d) ?


Thanks in advance,

Osiander


-- 
osiander at 24on.cc
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osiander at 24on.cc writes:
> Hi community,
> 
> Seems I am too stupid:
> 
> I try to calculate with power.t.test() an example from the statistics
textbook [Jerrold H. Zar, Biostatistical Analysis, 2nd edition, Prentice Hall,
1984, Chapter 8, Exercise 8.4, (page 120)]:
Ah, this is not a standard sample size calculation, but a pedagogical
exercise. t tests and power.t.test are irrelevant for this.
 
> -------
> A sample of size 18 has a mean of 13.55cm and a variance of 6.4512cm^2.
> (a) Calculate the 95% confidence interval for the population mean.
> (b) How large a sample would have to be taken from this population to
estimate ? to within 1.00cm,
>  with 95% confidence?
> (c) 2.00cm with 95% confidence?
> (d) 2.00cm with 99% confidence?
> 
> Result should be - according to the answers appendix - (a) 13.55 +- 1.26cm
(b) n=29, (c) n=10, (d) n=11
> -------
> 
> Here my questions:
> 
> (1) which functions in R can be used to solve (a)
> (2) what has to be the syntax to answer (b), (c) and (d) ?
Hmmm, now are you trying to learn R for yourself by trying it on some
textbook examples, or are you trying to get us to help with your
homework?

Either way: You'll need the formula for the confidence interval
(presumably a couple of pages earlier in the book). The qnorm()
function gives you quantiles of the Normal distibution, the rest
should be basic algebra.

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
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