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# RE: st: RE: RE: comparing different means using ttest

 From DE SOUZA Eric <[email protected]> To "'[email protected]'" <[email protected]> Subject RE: st: RE: RE: comparing different means using ttest Date Thu, 16 Dec 2010 22:06:02 +0100

```I ought to have added that there may be other problems. Quarterly data, if not seasonally adjusted, will display seasonal patterns. In this case, the growth rates would have to be calculated across four quarters.

Eric de Souza
College of Europe
BE-8000 Brugge (Bruges)
Belgium

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of DE SOUZA Eric
Sent: 16 December 2010 21:46
To: '[email protected]'
Subject: RE: st: RE: RE: comparing different means using ttest

It does, because it simply avoids the starting point of David Lempert which in my opinion is a false start: regressing GDP levels on a time trend will get you nowhere. If David is interested testing the equality of GDP growth rates across two time periods, you pool the data, calculate the GDP growth rate and regress this variable on two dummy (binary) variables for each time period. In order to avoid perfect collinearit you drop one of the two dummies and test whether the coefficient on the other is equal to zero.

Eric

Eric de Souza
College of Europe
BE-8000 Brugge (Bruges)
Belgium

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Steven Samuels
Sent: 16 December 2010 21:17
To: [email protected]
Subject: Re: st: RE: RE: comparing different means using ttest

But. Eric, I don't think that pooling will solve the dependence issues that Nick mentioned.

Steve

[email protected]

On Dec 16, 2010, at 1:26 PM, DE SOUZA Eric wrote:

Reply to original post, which once again I have deleted !

Why not just pool your data and regress %GDP-growth on a dummy
(binary) variable (and a constant, of course) which takes the value of one for one of the two sub-samples and zero for the other; and test whether the coefficient on the dummy is significantly different from zero (or examine its confidence interval) ?
You can robustify for heteroscedasticity.

Eric de Souza
College of Europe
Dyver 11
BE-8000 Brugge (Bruges)
Belgium

-----Original Message-----
From: [email protected] [mailto:[email protected]
] On Behalf Of Nick Cox
Sent: 16 December 2010 19:17
To: '[email protected]'
Subject: <POSSIBLE SPAM>st: RE: RE: comparing different means using ttest

A senior Stata user, who might not want to be named, pointed out the counter-example of a Poisson variable. Clearly correct: if you know that your variable is Poisson, then the mean is also the variance.

Nick
[email protected]

Nick Cox

[...]
any more than the mean of anything tells you about its variability.
[...]

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