[R] delta and sd parameters for power.t.test

[David Winsemius]

> On Dec 14, 2017, at 2:26 PM, Lasse Kliemann <lasse at lassekliemann.de> wrote:
> 
> What is the rationale behind having both the delta and sd parameters for
> the power.t.test function?

One is the standard deviation of the hypothesized data (or pooled sd in the case of two sample) under the "alternative" and one is the mean of that data (or equivalently the differences if this is a two-sample test).

> For the relevant noncentrality parameter, we
> only need the ratio delta/sd. If my effect size is given as Cohen's d,
> then I only got that ratio and not sd.
> 

If the data has been "standardized", then Cohen's d could be given to the function as the value for delta since the sd default is 1.


> As far as I see, in such a case, I can specify delta=d and leave sd at
> its default value 1. Is this correct or am I missing something?
> 

Actually it sounds as though we are missing something. The power.t.test function makes no mention of Cohen's d or effect size. Is this question in response to a homework assignment about which we have not been informed?

Further questions should have some actual R code to make this an on-topic discussion for Rhelp.


> Thanks.
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David Winsemius
Alameda, CA, USA

'Any technology distinguishable from magic is insufficiently advanced.'   -Gehm's Corollary to Clarke's Third Law