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Re: st: Graphs With Log Scale: A Bug?


From   "Vladimir V. Dashkeyev" <dashkeyev@iet.ru>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Graphs With Log Scale: A Bug?
Date   Wed, 28 May 2008 15:45:42 +0400

Martin,

Thanks for the reply. I should have emphasized in the first message,
that I run -lfitci- of X on ln(Y) in both scenarios. The difference is
in the scatter plot. In the first scenario I use ln(Y), and in the
second -- Y with log scale option. I expected to get the same linear
prediction line and the same scatter plot.

But after I posted that question, I compared the graphs once again and
realized that the real problem is with the Y axis scale. If I draw a
scatter and prediction line on the same Y axis, everything is fine.
Yet if I draw the same scatter with 2 Y axes I get different range of
values on Y1 and Y2 axes. I need two Y axes for overlaid drawing of
the scatter with -yscale (log)- option and linear prediction of
X-ln(Y). Setting range on both axes to the same values did not help.
They are very close but still shifted a bit. So the arrangement of
observations and prediction line is not correct. So it's not a bug,
but still a problem I have to solve.

Is there any way to "tie" axis Y1 with axis Y2?

Thanks,
Vladimir

On Wed, May 28, 2008 at 2:24 PM, Maarten buis <maartenbuis@yahoo.co.uk> wrote:
> --- "Vladimir V. Dashkeyev" <dashkeyev@iet.ru> wrote:
>> I drew a two-way plot with a linear prediction line -lfitci- of X on
>> natural logarithm of Y. Next, I drew the plot of X on Y with log
>> scale option -yscale(log)-.
>>
>> To my surprise regression line changed its slope. The slope is
>> greater with the -yscale(log)- option. I used the same X axis and
>> the second Y-axis for the linear prediction graph .
>> Is this a bug or am I doing something wrong?
>
> This is not a bug: in the first scenario you are thinking that there is
> a linear relationship between ln(Y) and X and you are showing the
> predictions, while in the second scenario you are thingking that there
> is a linear relationship between Y and X and then transforme the
> predictions to a log scale. So the results are different because the
> models are different.
>
> -- Maarten
>
>
> -----------------------------------------
> Maarten L. Buis
> Department of Social Research Methodology
> Vrije Universiteit Amsterdam
> Boelelaan 1081
> 1081 HV Amsterdam
> The Netherlands
>
> visiting address:
> Buitenveldertselaan 3 (Metropolitan), room Z434
>
> +31 20 5986715
>
> http://home.fsw.vu.nl/m.buis/
> -----------------------------------------
>
>
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