[R] power.t.test threading on 'power'

[Stephen Kennedy]

Thanks.  I’ve been getting some nice workarounds.  I was pleasantly surprised when the function ‘threaded’ on the number of samples, but I guess that was not really part of the design, just an accident.

Best,

Steve



On Oct 1, 2014, at 6:15 PM, Uwe Ligges <ligges at statistik.tu-dortmund.de> wrote:

On 01.10.2014 14:29, Stephen Kennedy wrote:
> Simple question.  A vector of ‘number of observations’ can be input to power.t.test, and a vector of ‘power’ s is output.  But, inputting a vector of powers generates an error.  Am I missing something?
> 
> Vector of ’n’ s
> 
> power.t.test(n=c(28,29,30), delta=2, sd=3, sig.level=0.05, type="two.sample", alternative="one.sided")
> 
>      Two-sample t test power calculation
> 
>               n = 28, 29, 30
>           delta = 2
>              sd = 3
>       sig.level = 0.05
>           power = 0.7933594, 0.8058963, 0.8177506
>     alternative = one.sided
> 
> NOTE: n is number in *each* group
> 
> 
> 
> Vector of ‘power’ s
> 
> power.t.test(power=c(0.7,0.8,0.9), delta=2, sd=3, sig.level=0.05, type="two.sample", alternative="one.sided")
> Error in uniroot(function(n) eval(p.body) - power, c(2, 1e+07)) :
>   f() values at end points not of opposite sign
> In addition: Warning messages:
> 1: In if (is.na(f.lower)) stop("f.lower = f(lower) is NA") :
>   the condition has length > 1 and only the first element will be used
> 2: In if (is.na(f.upper)) stop("f.upper = f(upper) is NA") :
>   the condition has length > 1 and only the first element will be used
> 

power.t.test oes not work on vectors in general.

Here, you want:

lapply(c(0.7, 0.8, 0.9),
   function(power)
       power.t.test(power=power, delta=2, sd=3, sig.level=0.05,
                    type="two.sample", alternative="one.sided")
)

Best,
Uwe Ligges




> 
> Thanks,
> 
> Steve
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