Homework 5

Doug Hemken

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)

``````