# Breaking the samplesize package from CRAN

3 messages
Open this post in threaded view
|

## Breaking the samplesize package from CRAN

 Hello all, I am using the samplesize package (n.ttest function) to calculate number of samples per group power analysis (t-tests with unequal variance). I can break this n.ttest function from the samplesize package, depending on the standard deviations I input. This works very good. n.ttest(sd1 = 0.35, sd2 = 0.22 , variance = "unequal") # outputs \$`Total sample size` [1] 8 \$`Sample size group 1` [1] 5 \$`sample size group 2` [1] 3 Warning message: In n.ttest(sd1 = 0.35, sd2 = 0.22, variance = "unequal") :   Arguments -fraction- and -k- are not used, when variances are unequal The warnings are fine and all is good. But if I run it again with. n.ttest(sd1 = 1.68, sd2 = 0.28 , variance = "unequal") # outputs Error in while (n.start <= n.temp) { :   missing value where TRUE/FALSE needed In addition: Warning messages: 1: In n.ttest(sd1 = 1.68, sd2 = 0.28, variance = "unequal") :   Arguments -fraction- and -k- are not used, when variances are unequal 2: In qt(conf.level, df = df_approx) : NaNs produced 3: In qt(power, df = df_approx) : NaNs produced It breaks. The first obvious thing is that the standard deviations are a lot different in the 2nd example that breaks, compared with the first run. Checking the code myself, I can see it breaks down when the variable "df_approx" becomes a negative number, in a while loop from the n.ttest function. Exert of the code I am talking about. while (n.start <= n.temp) {     n.start <- n1 + n2 + 1     n1 <- n.start/(1 + k)     n2 <- (k * n.start)/(1 + k)     df_approx <- 1/((gamma)^2/(n1 - 1) + (1 - gamma)^2/(n2 - 1))   # this calculation becomes negative and breaks subsequently     tkrit.alpha <- qt(conf.level, df = df_approx)     tkrit.beta <- qt(power, df = df_approx)     n.temp <- ((tkrit.alpha + tkrit.beta)^2)/(c^2) } I can hard code df_approx to be an absolute value but I don't know if that messes up the statistics. Can anyone help or any ideas? How to fix? John. ______________________________________________ [hidden email] mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.