thanks, i am afraid that i mis-understand it before
thanks again
----- Original Message -----
From: "Richard Williams" <[email protected]>
To: <[email protected]>
Sent: Monday, October 11, 2004 9:38 PM
Subject: Re: st: Re: Re: ST: one-tailed test
> At 09:09 PM 10/11/2004 +0800, Levy Lee wrote:
> >thanks again,
> >
> >but i do not think this is precise, are there other way to go?
>
> No, unless I misunderstand you, it is exactly right. Consider a
regression
> where you have a single dichotomous independent variable:
>
> . reg income race
>
> Source | SS df MS Number of obs =
20
> -------------+------------------------------ F( 1, 18) =
8.55
> Model | 586.444492 1 586.444492 Prob > F =
0.0090
> Residual | 1233.98098 18 68.5544991 R-squared =
0.3221
> -------------+------------------------------ Adj R-squared =
0.2845
> Total | 1820.42548 19 95.8118671 Root MSE =
8.2798
>
> --------------------------------------------------------------------------
----
> income | Coef. Std. Err. t P>|t| [95% Conf.
Interval]
> -------------+------------------------------------------------------------
----
> race | -10.83 3.702823 -2.92
0.009 -18.60934 -3.050657
> _cons | 29.83 2.618291 11.39 0.000 24.32917
35.33083
> --------------------------------------------------------------------------
----
>
> The reported 2-tailed significance of race is .009. You can do an
> equivalent analysis with the t-test command:
>
> . ttest income, by(race)
>
> Two-sample t test with equal variances
>
> --------------------------------------------------------------------------
----
> Group | Obs Mean Std. Err. Std. Dev. [95% Conf.
Interval]
> ---------+----------------------------------------------------------------
----
> white | 10 29.83 2.639573 8.347062 23.85887
35.80113
> black | 10 19 2.596836 8.211915 13.12555
24.87445
> ---------+----------------------------------------------------------------
----
> combined | 20 24.415 2.188742 9.788354 19.83391
28.99609
> ---------+----------------------------------------------------------------
----
> diff | 10.83 3.702823 3.050657
18.60934
> --------------------------------------------------------------------------
----
> Degrees of freedom: 18
>
> Ho: mean(white) - mean(black) = diff = 0
>
> Ha: diff < 0 Ha: diff != 0 Ha: diff > 0
> t = 2.9248 t = 2.9248 t = 2.9248
> P < t = 0.9955 P > |t| = 0.0090 P > t = 0.0045
>
> The 2-tailed significance is again .0090; the 1-tailed significance is
> exactly half that, .0045. (The only difference is that the t-test
> basically reversed the 2 groups from what the regression had done.)
>
>
> -------------------------------------------
> Richard Williams, Notre Dame Dept of Sociology
> OFFICE: (574)631-6668, (574)631-6463
> FAX: (574)288-4373
> HOME: (574)289-5227
> EMAIL: [email protected]
> WWW (personal): http://www.nd.edu/~rwilliam
> WWW (department): http://www.nd.edu/~soc
>
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