# st: R: Mata question

 From "Carlo Lazzaro" To Subject st: R: Mata question Date Sat, 4 Jul 2009 10:45:45 +0200

```Dear Christian,
the following is a temptative answer to your query, not in Mata language.
Let's assume we have 20 + 20 sample means and sds (sample size: 10
observations each) drawn from 2 different populations (I mean, 20 from
population1 and 20 from population 2). Sample means and sds drawn from
population 1 are labelled a and c. Sample means and sds drawn from
population 2 are labelled b and d. Ttests are performed under the equal
variance assumption. Example stops at Student t retrieving and storing for
each ttest performed (two-sided p-value was not included in the Stata .do
file).

set obs 20
g a==2*uniform()
g b=1*uniform()
g  c=.03*uniform()
g d =.03*uniform()
g double student_t=.
qui forval i = 1/20 {
g t`i' = (a[`i']-b[`i'])/(c[`i']^2/10+d[`i']^2/10)^.5
sum t`i', meanonly
replace  student_t=r(mean) in `i'
drop t`i'
}

Not that elegant, but I do hope this help.

Kind Regards,
Carlo
-----Messaggio originale-----
Da: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] Per conto di Christian
Bustamante
Inviato: sabato 4 luglio 2009 9.15
A: Statalist
Oggetto: st: Mata question

Hi all

I'm trying to do a loot with a nested -ttesti- but I couldn't do it. I
have two vector with different means (m1 and m2) and another two with
each mean's standard deviation (s1 and s2). This vectors are quite
large, so do -ttesti- element for element could be very costly. Also,
I have each sample's size on two variables: n1 and n2. All this values
are obtained from a logistic regresion. After run -ttesti- I want to
save the t-statistic -r(t)- and the probability for two-sided p-value
-r(p)-. I'm doing something like this:

forvalues i=1/size(m1) {    /* how can i get vector's size? */
ttesti n1 m1[1,`i'] s1[1,`i'] n1 m1[1,`i'] s1[1,`i']
mat tvalues[1,`i']=r(t)
mat pvalues[1,`i']=r(p)
}

But appears a lot of errors: 1) n1 should be integer. 2) 'm1' found
where number expected. 3) varlist not allowed.
How can I solve this?

Thanks

--
CdeB
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