On Thu, May 6, 2010 at 6:09 PM, Greg Snow <greg.s...@imail.org> wrote:
> Because if you use the sample standard deviation then it is a t test not a > z test. > I'm doubting that seriously... You calculate normalized Z-values by substracting the sample mean and dividing by the sample sd. So Thomas is correct. It becomes a Z-test since you compare these normalized Z-values with the Z distribution, instead of the (more appropriate) T-distribution. The T-distribution is essentially a Z-distribution that is corrected for the finite sample size. In Asymptopia, the Z and T distribution are identical. > > -- > Gregory (Greg) L. Snow Ph.D. > Statistical Data Center > Intermountain Healthcare > greg.s...@imail.org > 801.408.8111 > > > > -----Original Message----- > > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r- > > project.org] On Behalf Of Thomas Roth > > Sent: Thursday, May 06, 2010 1:58 AM > > To: 'level'; r-help@r-project.org > > Subject: Re: [R] P values > > > > Why > > > > s = 1 > > > > ## > > > > s = sd(A) #? > > > > > > -----Ursprüngliche Nachricht----- > > Von: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] > > Im > > Auftrag von level > > Gesendet: Mittwoch, 5. Mai 2010 22:41 > > An: r-help@r-project.org > > Betreff: [R] P values > > > > > > How do u calculated p values for a z test.. > > > > so far i ve done this > > > > A = read.table("cw3_data.txt") > > > > xbar = mean(A) > > s = 1 > > n = 20 > > mu = 0 > > > > z.test = (xbar-mu)/(s/sqrt(n)) > > > > p.value = pnorm(abs(z.test)) > > > > error = qnorm(0.99)*s/sqrt(n) > > left = xbar - error > > right = xbar + error > > > > and have got values off of it...but the values for p dont match up with > > other sites that i have used to check it agaisnt..such as wolfram alpha > > value i got was 0.7335039 when wolfram got 0.533.. any ideas where i > > ve > > gone wrong? cheers > > -- > > View this message in context: > > http://r.789695.n4.nabble.com/P-values-tp2131646p2131646.html > > Sent from the R help mailing list archive at Nabble.com. > > > > ______________________________________________ > > R-help@r-project.org mailing list > > https://stat.ethz.ch/mailman/listinfo/r-help > > PLEASE do read the posting guide http://www.R-project.org/posting- > > guide.html > > and provide commented, minimal, self-contained, reproducible code. > > > > ______________________________________________ > > R-help@r-project.org mailing list > > https://stat.ethz.ch/mailman/listinfo/r-help > > PLEASE do read the posting guide http://www.R-project.org/posting- > > guide.html > > and provide commented, minimal, self-contained, reproducible code. > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide > http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > -- Joris Meys Statistical Consultant Ghent University Faculty of Bioscience Engineering Department of Applied mathematics, biometrics and process control Coupure Links 653 B-9000 Gent tel : +32 9 264 59 87 joris.m...@ugent.be ------------------------------- Disclaimer : http://helpdesk.ugent.be/e-maildisclaimer.php [[alternative HTML version deleted]]
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