Hi, I do want to use the Mann-Whitney test which ranks my data and then uses those ranks rather than the actual data.
Here is the R code i am using: group1<- c(1.34,1.47,1.48,1.49,1.62,1.67,1.7,1.7,1.7,1.73,1.81,1.84,1.9,1.96,2,2,2.19,2.29,2.29,2.41,2.41,2.46,2.5,2.6,2.8,2.8,3.07,3.3) > group2<- c(0.98,1.18,1.25,1.33,1.38,1.4,1.49,1.57,1.72,1.75,1.8,1.82,1.86,1.9,1.97,2.04,2.14,2.18,2.49,2.5,2.55,2.57,2.64,2.73,2.77,2.9,2.94,NA) > result <- wilcox.test(group1, group2, paired=FALSE, conf.level = 0.95, na.action) paired = FALSE so that the Wilcoxon rank sum test which is equivalent to the Mann-Whitney test is used (my samples are NOT paired). conf.level = 0.95 to specify the confidence level na.action is used because i have a NA value (i suspect i am not using na.action in the correct manner) When i use this code i get the following error message: Error in arg == choices : comparison (1) is possible only for atomic and list types When i use this code: group1<- c(1.34,1.47,1.48,1.49,1.62,1.67,1.7,1.7,1.7,1.73,1.81,1.84,1.9,1.96,2,2,2.19,2.29,2.29,2.41,2.41,2.46,2.5,2.6,2.8,2.8,3.07,3.3) > group2<- c(0.98,1.18,1.25,1.33,1.38,1.4,1.49,1.57,1.72,1.75,1.8,1.82,1.86,1.9,1.97,2.04,2.14,2.18,2.49,2.5,2.55,2.57,2.64,2.73,2.77,2.9,2.94,NA) > result <- wilcox.test(group1, group2, paired=FALSE, conf.level = 0.95) I get the following result: Wilcoxon rank sum test with continuity correction data: group1 and group2 W = 405.5, p-value = 0.6494 alternative hypothesis: true location shift is not equal to 0 Warning message: cannot compute exact p-value with ties in: wilcox.test.default(group1, group2, paired = FALSE, conf.level = 0.95) The W value here is 405.5 with a p-value of 0.6494 in SPSS, i am ranking my data and then performing a Mann-Whitney U by selecting analyze - non-parametric tests - 2 independent samples and then checking off the Mann-Whitney U test. For the Mann-Whitney test in SPSS i am gettting the following results: Mann-Whitney U = 350.5 2- tailed p value = 0.643 I think maybe the descrepancy has to do with the specification of the NA values in R, but i'm not sure. If anyone has any suggestions, please let me know! I hope i have provided enough information to convey my problem. Thank-you, Nat __________________ Natalie, It's best to provide at least a sample of your data. Your field names suggest that your data might be collected in units of mm^2 or some similar measurement of area. Why do you want to use Mann-Whitney, which will rank your data and then use those ranks rather than your actual data? Unless your sample is quite small, why not use a two sample t-test? Also,are your samples paired? If they aren't, did you use the "paired = FALSE" option? JWDougherty ______________________________________________ R-help@stat.math.ethz.ch 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. ------------------------------------------------------------------------------------------------------------------------ This communication is intended for the use of the recipient to which it is addressed, and may contain confidential, personal, and or privileged information. Please contact the sender immediately if you are not the intended recipient of this communication, and do not copy, distribute, or take action relying on it. Any communication received in error, or subsequent reply, should be deleted or destroyed. ------------------------------------------------------------------------------------------------------------------------ This communication is intended for the use of the recipient to which it is addressed, and may contain confidential, personal, and or privileged information. Please contact the sender immediately if you are not the intended recipient of this communication, and do not copy, distribute, or take action relying on it. Any communication received in error, or subsequent reply, should be deleted or destroyed. ------------------------------------------------------------------------------------------------------------------------ This communication is intended for the use of the recipient to which it is addressed, and may contain confidential, personal, and or privileged information. Please contact the sender immediately if you are not the intended recipient of this communication, and do not copy, distribute, or take action relying on it. Any communication received in error, or subsequent reply, should be deleted or destroyed. [[alternative HTML version deleted]] ______________________________________________ R-help@stat.math.ethz.ch 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.
