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st: testing group means
I am testing if 4 group means are different from zero. Stata is using the
std dev of each group to perform the t tests. This causes that the "Power &
Pipelines" group mean is not diffrent from 0 with a mean of -0.0099, but the
REIT group is, with a mean of -0.0044. I would prefer to use the overall
std dev. to make the t tests. How do I tell Stata to use the overall SD?
Thanks.
by trust_type, sort : ttest meanmonthlyar == 0
-> trust_type = Power & Pipelines
One-sample t test
------------------------------------------------------------------------------
Variable | Obs Mean Std. Err. Std. Dev. [95% Conf.
Interval]
---------+--------------------------------------------------------------------
meanmo~r | 18 -.0099333 .0063121 .02678 -.0232507
.003384
------------------------------------------------------------------------------
mean = mean(meanmonthlyar) t
-1.5737
Ho: mean = 0 degrees of freedom =
17
Ha: mean < 0 Ha: mean != 0 Ha: mean > 0
Pr(T < t) = 0.0670 Pr(|T| > |t|) = 0.1340 Pr(T > t) =
0.9330
-----------------------------------------------------------------------------------------------------------------------------
-> trust_type = REIT
One-sample t test
------------------------------------------------------------------------------
Variable | Obs Mean Std. Err. Std. Dev. [95% Conf.
Interval]
---------+--------------------------------------------------------------------
meanmo~r | 25 -.004356 .0021169 .0105847 -.0087252
.0000131
------------------------------------------------------------------------------
mean = mean(meanmonthlyar) t
-2.0577
Ho: mean = 0 degrees of freedom =
24
Ha: mean < 0 Ha: mean != 0 Ha: mean > 0
Pr(T < t) = 0.0253 Pr(|T| > |t|) = 0.0506 Pr(T > t) =
0.9747
-----------------------------------------------------------------------------------------------------------------------------
-> trust_type = Business Trust
One-sample t test
------------------------------------------------------------------------------
Variable | Obs Mean Std. Err. Std. Dev. [95% Conf.
Interval]
---------+--------------------------------------------------------------------
meanmo~r | 95 .0040821 .0055586 .0541783 -.0069546
.0151188
------------------------------------------------------------------------------
mean = mean(meanmonthlyar) t =
0.7344
Ho: mean = 0 degrees of freedom =
94
Ha: mean < 0 Ha: mean != 0 Ha: mean > 0
Pr(T < t) = 0.7677 Pr(|T| > |t|) = 0.4645 Pr(T > t) =
0.2323
-----------------------------------------------------------------------------------------------------------------------------
-> trust_type = Energy Trust
One-sample t test
------------------------------------------------------------------------------
Variable | Obs Mean Std. Err. Std. Dev. [95% Conf.
Interval]
---------+--------------------------------------------------------------------
meanmo~r | 33 .0057545 .005595 .0321408 -.0056421
.0171512
------------------------------------------------------------------------------
mean = mean(meanmonthlyar) t =
1.0285
Ho: mean = 0 degrees of freedom =
32
Ha: mean < 0 Ha: mean != 0 Ha: mean > 0
Pr(T < t) = 0.8443 Pr(|T| > |t|) = 0.3114 Pr(T > t) =
0.1557
-----------------------------------------------------------------------------------------------------------------------------
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