Dear Bert,
Thanks for your answer, I already wrote to the maintainer/author of
samplesize, Ralph Scherer, on Thu, Apr 19, 2018 but still have no
answer.
Does anyone have any ideas? Thank you.
John.
On 26 July 2018 at 20:18, Bert Gunter <bgunter.4...@gmail.com> wrote:
> Suggest you contact the package maintainer.
>
> ?maintainer
>
> Cheers,
> Bert
>
> Bert Gunter
>
> "The trouble with having an open mind is that people keep coming along and
> sticking things into it."
> -- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )
>
> On Thu, Jul 26, 2018 at 9:49 AM, john matthew via R-help
> <r-help@r-project.org> wrote:
>>
>> 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.
>>
>> ______________________________________________
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>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
>
______________________________________________
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and provide commented, minimal, self-contained, reproducible code.
