Illustrate a two-group t-test using Stata. Illustrate with a box plot.
First, simulate some data.
. clear
. set obs 2
number of observations (_N) was 0, now 2
. generate group = _n - 1
. expand = 15
(28 observations created)
. generate y = rnormal(group)
Let’s check the data we created.
. table group, contents(freq mean y sd y semean y) row
----------------------------------------------------------
group | Freq. mean(y) sd(y) sem(y)
----------+-----------------------------------------------
0 | 15 -.0062374 .6081133 .1570142
1 | 15 .7164829 .8867842 .2289667
|
Total | 30 .3551227 .8326126 .1520136
----------------------------------------------------------
Next, do the t-test.
. ttest y, by(group)
Two-sample t test with equal variances
------------------------------------------------------------------------------
Group | Obs Mean Std. Err. Std. Dev. [95% Conf. Interval]
---------+--------------------------------------------------------------------
0 | 15 -.0062374 .1570142 .6081134 -.3429993 .3305246
1 | 15 .7164829 .2289667 .8867842 .2253982 1.207568
---------+--------------------------------------------------------------------
combined | 30 .3551227 .1520136 .8326126 .0442201 .6660254
---------+--------------------------------------------------------------------
diff | -.7227203 .2776314 -1.291422 -.1540181
------------------------------------------------------------------------------
diff = mean(0) - mean(1) t = -2.6032
Ho: diff = 0 degrees of freedom = 28
Ha: diff < 0 Ha: diff != 0 Ha: diff > 0
Pr(T < t) = 0.0073 Pr(|T| > |t|) = 0.0146 Pr(T > t) = 0.9927
Finally, a box plot.
. graph box y, by(group)
