[R-sig-eco] Understanding of P-values from rankings

[Alexandre F. Souza]

Hi,

For a simulation of 1000 runs and a threshold of 0.05:

For a two-tailed test all ranks < 25 or > 975 would be significant

For a one-tailed test all ranks < 50 would be significant (considering a
"less-than" one-tailed hypothesis).

I think I got it now.

Thank you,

Best,

Alexandre



2015-04-08 14:50 GMT-03:00 Alexandre F. Souza <
alexsouza.cb.ufrn.br at gmail.com>:

>
> Dear friends,
>
> I have fun a series of null models of tree species traits (e.g., height)
> in permanent forest plots using the code found in Kraft's web page (
> http://life.umd.edu/biology/kraftlab/Code_files/trait_tests.R). Null
> distributions of trait metrics were generated by creating 999 null
> communities of equal richness to the sample quadrat by drawing species at
> random from our entire trait database, weighted by plot-wide occurrence
> (the number of quadrats in which each species is found).
>
> For each of 245 plots, the output consists in the following output:
>
> test.richness 11 reps 999 mean_rank 972 obs_mean 0.850482 null_mean_mean
> 0.816427 null_mean_sd 0.019774
> Where mean_rank means the ranking of the observed mean.
>
> Now I need to obtain a P-value from the ranking information. I understand
> that a one-tailed P-value would be = observed rank/(number of runs + 1),
> and two-tailed P values should be twice this value, as found in the manual
> of Phylocom. Having 999 runs this gives almost equal Ps as the ranks of the
> means, with P = 0.972 for a one-tailed test in this case. However, a
> two-tailed test would yield a P-value > 1.0 in this case.
>
> How do I use the rank of the mean with the number of runs to obtain a
> one-tailed and two-tailed value?
>
> Sorry for the introductory level of the question, but it is for a good
> cause.
>
> Best,
>
> Alexandre
>
> --
> Dr. Alexandre F. Souza
> Professor Ajunto III
> Universidade Federal do Rio Grande do Norte
> CB, Departamento de Ecologia
> Campus Universitário - Lagoa Nova
> 59072-970 - Natal, RN - Brasil
> lattes: lattes.cnpq.br/7844758818522706
> http://www.docente.ufrn.br/alexsouza
>



-- 
Dr. Alexandre F. Souza
Professor Ajunto III
Universidade Federal do Rio Grande do Norte
CB, Departamento de Ecologia
Campus Universitário - Lagoa Nova
59072-970 - Natal, RN - Brasil
lattes: lattes.cnpq.br/7844758818522706
http://www.docente.ufrn.br/alexsouza

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