Can anyone explain why -ttest- without the unequal option does not report
the pooled standard deviation. In the output below, for example, it is
possible one might erroneously conclude that the pooled standard deviation
was 5.785503.
Paul Dickman ([email protected])
. use "C:\Program Files\stata7\auto"
(1978 Automobile Data)
. ttest mpg, by(foreign)
Two-sample t test with equal variances
----------------------------------------------------------------------------
--
Group | Obs Mean Std. Err. Std. Dev. [95% Conf.
Interval]
---------+------------------------------------------------------------------
--
Domestic | 52 19.82692 .657777 4.743297 18.50638
21.14747
Foreign | 22 24.77273 1.40951 6.611187 21.84149
27.70396
---------+------------------------------------------------------------------
--
combined | 74 21.2973 .6725511 5.785503 19.9569
22.63769
---------+------------------------------------------------------------------
--
diff | -4.945804
1.362162 -7.661225 -2.230384
----------------------------------------------------------------------------
--
Degrees of freedom: 72
Ho: mean(Domestic) - mean(Foreign) = diff = 0
Ha: diff < 0 Ha: diff ~= 0 Ha: diff > 0
t = -3.6308 t = -3.6308 t = -3.6308
P < t = 0.0003 P > |t| = 0.0005 P > t = 0.9997
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