st: R: Test for statistical significance - flag Stata 9.2/SE

["Carlo Lazzaro" <carlo.lazzaro@tin.it>]

Cornelius may want to calculate a bootstrap two-sided p-value instead of
rely upon the results of a probably bad-behaving unpaired ttest (I would
assume that the manganese serum distribution in humans departs from
normality):
--------------- code begins------------------------------------------
set obs 100
g Manga_1=uniform()*1 
g Manga_2=uniform()*1 
ttest Manga_1 == Manga_2, unpaired unequal
return list
scalar tobs=r(t)
replace Manga_1=Manga_1-r(mu_1)+(r(mu_1)+r(mu_2))/2
replace Manga_2=Manga_2-r(mu_2)+(r(mu_1)+r(mu_2))/2
bootstrap r(t), reps(10000) nodots saving(C:\Documents and
Settings\carlo\Desktop\Manga_boot.dta, every(1) double replace) : ttest
Manga_1 == Manga_2, unpaired unequal
use "C:\Documents and Settings\carlo\Desktop\Manga_boot.dta", clear
g indicator =abs(_bs_1)>=abs(scalar(tobs))
sum indicator, mean
di "p_bootstrap =" r(mean)
---------------code ends------------------------------------------
For further details on bootstrap procedures, the mandatory reference is:
Efron B, Tibshirani RJ. An introduction to the bootstrap. NewYork: Chapman
and Hall, 1993.

HTH and Kind Regards,
Carlo
-----Messaggio originale-----
Da: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] Per conto di Cornelius Nattey
Inviato: martedì 31 agosto 2010 16.25
A: statalist@hsphsun2.harvard.edu
Oggetto: st: Test for statistical significance

Dear All,
I have two datasets of manganese blood levels taken in August and
November 2005. They were from the same community but mainly different
people in the community. There may have been some repeats but I do not
know how many if there were any. I have treated them as independent and
not paired tests.

What test should be applied to see if there is a statistically sig
difference? Is there a difference?

Thanks very much
Cornelius


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