st: R: Logarithmic transformation in psm results

["Carlo Lazzaro" <carlo.lazzaro@tin.it>]

Sergio wrote:
"... using the original variable expressed in monetary units the value
corresponding to treated and controls not coincide with the transformed
values of the results in logarithms".

Without other details, and provided that your problem concerns
transformation from the log scale only, the first advice is to check that,
when back transforming to the original metric (I would assume that it is
arithmetic),the sample mean is actually obtained via the following formula:

arithmetic mean=exp(ln_mean+ln_variance/2)

where:

ln_mean is the mean on the log scale;
ln_variance is the variance on the log scale.


Kindest Regards,
Carlo
-----Messaggio originale-----
Da: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] Per conto di s
Inviato: giovedì 17 marzo 2011 11.55
A: statalist@hsphsun2.harvard.edu
Oggetto: st: Logarithmic transformation in psm results

Dear statalisters,

I'm  estimating propensity score matching (with psmatch2 routine) for a 
sample of firms participating in a
program. My outcome variable (turnover of the firm) is expressed in 
logarithm, and the results are similar to these:

----------------------------------------------------------------------------
------------------------------------------
           Sample                 Treated         Controls           
Difference         S.E.           T-stat
----------------------------+-----------------------------------------------
-----------------------------------------

         Unmatched         | 13.2005389   8.90516337   4.29537557   
.323484337    13.28
             ATT                 | 12.9789829    12.284779   .694203895  
    .303516206     2.29
----------------------------+-----------------------------------------------
-----------------------------------------

How can I transform the results in order to interpret this results in 
terms of the original values.
My doubt arise mainly because when I estimate the ATT using the original 
variable expressed in
monetary units the value corresponding to treated and controls not 
coincide with the transformed values
of the results in logarithms.

----------------------------------------------------------------------------
--------------------------------------------------
Sample                  Treated           Controls             
Difference         S.E.           T-stat
----------------------------+-----------------------------------------------
------------------------------------------------
Unmatched       |  3940905.5   642598.305       3298307.2        
266059.508    12.40
  ATT                  | 2687231.69  2091249.89       595981.797       
631105.497     0.94
----------------------------+-----------------------------------------------
------------------------------------------------

Thanks in advance for your help,

Sergio

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