st: testing group means

 From Claude Francoeur <[email protected]> To [email protected] Subject st: testing group means Date Sun, 07 Jan 2007 10:56:16 -0500

```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

-----------------------------------------------------------------------------------------------------------------------------

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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```