st: R: one-sample t test query

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

Dear Gwinyai,
I would suggest to move from -ttest- to -prtest-, as your testing
proportions (and not continuous variables):

prtest sex_baby == 0.515 //sex_baby is 1 for males

one-sample test of proportion               sex_baby: Number of obs =
32
----------------------------------------------------------------------------
--
    Variable |       Mean   Std. Err.                     [95% Conf.
Interval]
-------------+--------------------------------------------------------------
--
    sex_baby |       .375   .0855816                       .207263
.542737
----------------------------------------------------------------------------
--
    p = proportion(sex_baby)                                      z =
-1.5846
Ho: p = 0.515

    Ha: p < 0.515               Ha: p != 0.515                 Ha: p > 0.515
 Pr(Z < z) = 0.0565         Pr(|Z| > |z|) = 0.1130          Pr(Z > z) =
0.9435


Luckily enough, your conclusion is still valid: given your sample, there is
no evidence that 37.5% is significantly different from 51.5% if p < 0.05 is
considered significant.

Kind regards,
Carlo
-----Messaggio originale-----
Da: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] Per conto di Gwinyai Masukume
Inviato: lunedì 13 gennaio 2014 10:22
A: statalist
Oggetto: st: one-sample t test query

Hi Stata list,

I have a dataset with 32 observations (20 females and 12 males). Males thus
constitute 37.5% of the observations. Normally males should constitute 51.5%
of the observations. I want to test if the observed 37.5% males are
different from 51.5%. I have issued the following Stata command:

. ttest sex_baby = 0.515

One-sample t test
----------------------------------------------------------------------------
--
Variable |     Obs        Mean    Std. Err.   Std. Dev.   [95% Conf.
Interval]
---------+--------------------------------------------------------------
---------+------
sex_baby |      32        .375     .086951    .4918694    .1976622
.5523378
----------------------------------------------------------------------------
--
    mean = mean(sex_baby)                                         t =
-1.6101
Ho: mean = 0.515                                 degrees of freedom =
31

  Ha: mean < 0.515             Ha: mean != 0.515             Ha: mean >
0.515
 Pr(T < t) = 0.0588         Pr(|T| > |t|) = 0.1175          Pr(T > t) =
0.9412

Looking at the two-tailed p-value, I conclude that 37.5% is not
significantly different from 51.5% if p < 0.05 is considered significant.
Have I used the correct Stata command and is my interpretation correct?

Thanks,
Gwinyai
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