Discrepency between confidence intervals from t.test and computed manually -- why?

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Discrepency between confidence intervals from t.test and computed manually -- why?

 I am sure there is something simple here I am missing, so please bear with  me. It concerns the computation of the confidence interval for a population mean. The data are 125 measurements of Cs137 radation, a sample data set from Davis "Statistics and Data Analysis in Geology" 3rd ed. (CROATRAD.TXT) ------------------ method 1: using textbook definitions: mean \pm se_mean * t-value mu <- mean(Cs137); n <- length(Cs137) se.mean <- sqrt(var(Cs137)/n) # two-tail alphas alpha <- c(1, 5, 10, 20)/100 # t-values for each tail t.vals <- qt(1-(alpha/2), n-1) # name them for the respective alpha names(t.vals) <- alpha # low and high ends of the confidence interval round(ci.low <- mu - se.mean * t.vals, 2) round(ci.hi <- mu + se.mean * t.vals, 2) Output: 0.01 0.05  0.1  0.2 5.66 5.81 5.90 5.99 0.01 0.05  0.1  0.2 6.69 6.54 6.46 6.36 ----------------- So for the 95% confidence level I seem to get a CI of 5.81 .. 6.54 ------------------ method 2: using t.test.  I am not really testing for any specific mean, I just want the confidence interval of the mean, which t.test seems to give to me: Input: t.test(Cs137) Output:         One Sample t-test data:  Cs137 t = 11.5122, df = 124, p-value < 2.2e-16              <-- not relevant alternative hypothesis: true mean is not equal to 0   <-- not relevant 95 percent confidence interval:  5.115488 7.239712 sample estimates: mean of x    6.1776 ------------------------------ So with t.test I seem to get a CI of 5.12 .. 7.24 which is considerably wider than the directly computed interval 5.81 .. 6.54.  Perhaps I am mis-understanding the CI which t.test is reporting? Any help would be appreciated. Thank you. D G Rossiter Senior University Lecturer Department of Earth Systems Analysis (DESA) International Institute for Geo-Information Science and Earth Observation (ITC) Hengelosestraat 99 PO Box 6, 7500 AA Enschede, The Netherlands mailto:[hidden email],  Internet: http://www.itc.nl/personal/rossiter______________________________________________ [hidden email] mailing list https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
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Re: Discrepency between confidence intervals from t.test andcomputed manually -- why?

 for me your code works correctly with simulated data, e.g., Cs137 <- rexp(100, 1/6) mu <- mean(Cs137) n <- length(Cs137) se.mean <- sqrt(var(Cs137)/n) alpha <- c(1, 5, 10, 20)/100 t.vals <- qt(1 -(alpha/2), n-1) names(t.vals) <- alpha ci.low <- mu - se.mean * t.vals ci.hi <- mu + se.mean * t.vals ###################### rbind(ci.low, ci.hi) t.test(Cs137) maybe you overwrite somewhere the value of the vector Cs137. I hope it helps. Best, Dimitris ---- Dimitris Rizopoulos Ph.D. Student Biostatistical Centre School of Public Health Catholic University of Leuven Address: Kapucijnenvoer 35, Leuven, Belgium Tel: +32/(0)16/336899 Fax: +32/(0)16/337015 Web: http://www.med.kuleuven.be/biostat/     http://www.student.kuleuven.be/~m0390867/dimitris.htm----- Original Message ----- From: "David Rossiter" <[hidden email]> To: <[hidden email]> Sent: Wednesday, January 04, 2006 9:58 AM Subject: [R] Discrepency between confidence intervals from t.test andcomputed manually -- why? >I am sure there is something simple here I am missing, so please bear > with  me. > > It concerns the computation of the confidence interval for a > population > mean. > > The data are 125 measurements of Cs137 radation, a sample data set > from > Davis "Statistics and Data Analysis in Geology" 3rd ed. > (CROATRAD.TXT) > ------------------ > method 1: using textbook definitions: mean \pm se_mean * t-value > > mu <- mean(Cs137); n <- length(Cs137) > se.mean <- sqrt(var(Cs137)/n) > # two-tail alphas > alpha <- c(1, 5, 10, 20)/100 > # t-values for each tail > t.vals <- qt(1-(alpha/2), n-1) > # name them for the respective alpha > names(t.vals) <- alpha > # low and high ends of the confidence interval > round(ci.low <- mu - se.mean * t.vals, 2) > round(ci.hi <- mu + se.mean * t.vals, 2) > > Output: > 0.01 0.05  0.1  0.2 > 5.66 5.81 5.90 5.99 > > 0.01 0.05  0.1  0.2 > 6.69 6.54 6.46 6.36 > > ----------------- > > So for the 95% confidence level I seem to get a CI of 5.81 .. 6.54 > > ------------------ > method 2: using t.test.  I am not really testing for any specific > mean, > I just want the confidence interval of the mean, which t.test seems > to > give to me: > > Input: > t.test(Cs137) > > Output: > >        One Sample t-test > > data:  Cs137 > t = 11.5122, df = 124, p-value < 2.2e-16              <-- not > relevant > alternative hypothesis: true mean is not equal to 0   <-- not > relevant > 95 percent confidence interval: > 5.115488 7.239712 > sample estimates: > mean of x >   6.1776 > ------------------------------ > > So with t.test I seem to get a CI of 5.12 .. 7.24 which is > considerably > wider than the directly computed interval 5.81 .. 6.54.  Perhaps I > am > mis-understanding the CI which t.test is reporting? > > Any help would be appreciated. > > Thank you. > > D G Rossiter > Senior University Lecturer > Department of Earth Systems Analysis (DESA) > International Institute for Geo-Information Science and Earth > Observation (ITC) > Hengelosestraat 99 > PO Box 6, 7500 AA Enschede, The Netherlands > mailto:[hidden email],  Internet: > http://www.itc.nl/personal/rossiter> > ______________________________________________ > [hidden email] mailing list > https://stat.ethz.ch/mailman/listinfo/r-help> PLEASE do read the posting guide! > http://www.R-project.org/posting-guide.html> Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm______________________________________________ [hidden email] mailing list https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide! http://www.R-project.org/posting-guide.html