Re: st: RE: one-tailed tests

[Marcello Pagano <pagano@hsph.harvard.edu>]

m.p.

On 7/8/2010 10:01 AM, Nick Cox wrote:
> I don't know why anyone would want to do any test if you can get a
> confidence interval. After all, most of the best science should be
> quantitative, and a confidence interval is a report on the measurement
> scale of interest: it summarizes information, it indicates uncertainty
> and it can be compared with your qualitative ideas on sign or direction
> quite easily. If you think that something should be positive, or
> increase, or whatever, you can see whether the c.i. lies the correct
> side of zero, or whatever. In contrast, a yes-no test decision or even a
> P-value is a more cryptic and problematic reduction of the data.
>
> Actually, I don't know why anyone would get a confidence interval if you
> can put most or all of the data on a graph. After all, a good graph
> shows the main features of the data and detailed departures from those
> features, and it can give you ideas on what next to do.
>
> I have to estimate the extent to which Eric or Roger was talking tongue
> in cheek, and you will have to do the same about this.
>
> Nick
> n.j.cox@durham.ac.uk
>
> Eric Uslaner
>
> Bea Potter asked how to do one-tailed tests and Roger Newson responded:
> "I don't know why anybody would want to do a 1-tailed test, except if
> the
> distribution of the test statistic, under the null hypothesis, really IS
> one-tailed."
>
> I don't understand why anyone would do a two-tailed test.  If you have a
> theory, that theory is directional: The more A, the more B.  A
> two-tailed test is completely inappropriate for this.  E.g., in studies
> of voting behavior in political science, we usually posit that party
> identification is the dominant force behind voter choice.  So if you are
> a strong Democrat, you will vote Democratic.  A two-tailed test would
> imply that a strong Republican would be equally likely as a strong
> Democrat to vote Democratic.  This is plain silly.  If you don't have a
> theoretical focus, you don't have a research design worth submitting
> anywhere.
>
> So I don't understand why anyone does two-tailed tests and why Stata
> (together with other statistical programs) report them as defaults.
>
>
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