Hi list,Could you please help me to explain the following error messages with 't.test' in R Unix 2.1.1?  I don't see it in R under Windows (R 2.3.0) or Unix (R2.3.1).  Is it really due to the different R versions?Thanks,...TaoUnix session: (R.2.1.1)========================> R.version         _                       platform x86_64-unknown-linux-gnuarch     x86_64                  os       linux-gnu               system   x86_64, linux-gnu       status                           major    2                       minor    1.1                     year     2005                    month    06                      day      20                      language R                       > t.test(extra ~ group, data = sleep)        Welch Two Sample t-testdata:  extra by group t = -1.8608, df = 17.776, p-value = 0.0794alternative hypothesis: true difference in means is not equal to 0 95 percent confidence interval: -3.3654832  0.2054832 sample estimates:mean in group 1 mean in group 2            0.75            2.33 > t.test((1:6)~rep(1:2,each=3))Error in terms.formula(formula[-3]) : invalid model formula in ExtractVars> t.test(2^(1:6)~rep(1:2,each=3))Error in terms.formula(formula[-3]) : invalid power in formula> t.test(2^extra ~ group, data = sleep)Error in terms.formula(formula[-3]) : invalid power in formulaUnix session: (R 2.3.1)============================> R.version               _                         platform       x86_64-unknown-linux-gnu  arch           x86_64                    os             linux-gnu                 system         x86_64, linux-gnu         status                                   major          2                         minor          3.1                       year           2006                      month          06                        day            01                        svn rev        38247                     language       R                         version.string Version 2.3.1 (2006-06-01)> t.test(1:6~rep(1:2,each=3))        Welch Two Sample t-testdata:  1:6 by rep(1:2, each = 3) t = -3.6742, df = 4, p-value = 0.02131alternative hypothesis: true difference in means is not equal to 0 95 percent confidence interval: -5.266958 -0.733042 sample estimates:mean in group 1 mean in group 2               2               5 Windows session:===========================> R.version               _                         platform       i386-pc-mingw32           arch           i386                      os             mingw32                   system         i386, mingw32             status                                   major          2                         minor          3.0                       year           2006                      month          04                        day            24                        svn rev        37909                     language       R                         version.string Version 2.3.0 (2006-04-24)> help(t.test)> t.test(extra ~ group, data = sleep)        Welch Two Sample t-testdata:  extra by group t = -1.8608, df = 17.776, p-value = 0.0794alternative hypothesis: true difference in means is not equal to 0 95 percent confidence interval: -3.3654832  0.2054832 sample estimates:mean in group 1 mean in group 2            0.75            2.33 > t.test(2^extra ~ group, data = sleep)        Welch Two Sample t-testdata:  2^extra by group t = -1.6362, df = 10.718, p-value = 0.1308alternative hypothesis: true difference in means is not equal to 0 95 percent confidence interval: -18.641557   2.773344 sample estimates:mean in group 1 mean in group 2        3.448644       11.382751 > t.test(1:6~rep(1:2,each=3))        Welch Two Sample t-testdata:  1:6 by rep(1:2, each = 3) t = -3.6742, df = 4, p-value = 0.02131alternative hypothesis: true difference in means is not equal to 0 95 percent confidence interval: -5.266958 -0.733042 sample estimates:mean in group 1 mean in group 2               2               5 
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