st: Re: Re: difference between estimated ratios of x/y and ax/ay

["Zurab Sajaia" <zsajaia@hotmail.com>]

Thanks a lot Michael,

Of course it will be different, I'm so sorry for asking such a foolish 
question, I guess it's because I skipped breakfast this morning :)

Thanks,
Zurab




----- Original Message ----- 
From: "Michael Blasnik" <michael.blasnik@verizon.net>
To: <statalist@hsphsun2.harvard.edu>
Sent: Wednesday, October 17, 2007 12:44 PM
Subject: st: Re: difference between estimated ratios of x/y and ax/ay



> ..
> It's not rounding and its not an error.  The ratio estimate is the sum of 
> the numerator divided by the sum of the denominator (appropriately 
> weighted if weights are specified), it is not the average of the ratio. 
> So, unless the variable trunk is a constant, it will most likely lead to a 
> different estimate of the ratio.
>
> Michael Blasnik
>
> ----- Original Message ----- 
> From: "Zurab Sajaia" <zsajaia@hotmail.com>
> To: "statalist" <statalist@hsphsun2.harvard.edu>
> Sent: Wednesday, October 17, 2007 12:17 PM
> Subject: st: difference between estimated ratios of x/y and ax/ay
>
>
>
> > Dear All,
> >
> > I was estimating ratio of two variables (in fact, svy : ratio, but 
> > problem is the same for both) and after noticing that both numerator and 
> > denominator contained same component I decided to check whether canceling 
> > it would affect the results. To my surprise it did.  Lets consider this 
> > example:
> >
> > sysuse auto, clear
> >
> > . ratio weight / length
> > ...
> >    _ratio_1 |   16.06673   .2807194      15.50726     16.6262
> >
> > ...
> >
> > .generate double wa = weight * trunk
> > .generate double la  = length * trunk
> >
> > .ratio wa / la
> > ...
> >    _ratio_1 |   16.48413   .2797573      15.92658    17.04169
> > ...
> >
> >
> > Have anybody experienced similar case? does this thing come purely from 
> > numerical, rounding issues or there is something bigger (I can't think of 
> > any though)?
> >
> > Any insight would be appreciated.
> > Thanks
> > Zurab
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