# Re: st: Interpretation of regressionmodel of ln-transformed variable

 From Maarten buis To statalist@hsphsun2.harvard.edu Subject Re: st: Interpretation of regressionmodel of ln-transformed variable Date Mon, 3 Nov 2008 21:52:18 +0000 (GMT)

The way to model length of stay data is to use survival analysis and
not to use -regress-. Some online resources for learning about that
are:
http://www.iser.essex.ac.uk/teaching/degree/stephenj/ec968/
http://www.ats.ucla.edu/stat/stata/seminars/stata_survival/default.htm
http://home.fsw.vu.nl/m.buis/wp/survival.html

regarding the interpretation of a -regress- model after transforming
the dependent variable see:
http://www.stata.com/statalist/archive/2008-11/msg00039.html
http://www.stata.com/statalist/archive/2008-10/msg01362.html
http://www.stata.com/statalist/archive/2008-10/msg01364.html

Hope this helps,
Maarten

> I am analysisng the impact of laparoscopic surgery on hospital length
> of stay (LOS). The LOS is skewed and the median and 5-95 percentil
> range is exactly the same for laparoscopic and open surgery. The
> Mann-Whitney test is non significant.
>
> I want to model the LOS with some confounders (diagnosis at
> operation,
> sex, comorbidity, age). I have used linear regression on the
> ln-transformed LOS
>
> lnLOS        	Coef.	         Std. Err.	 t	P>t	         [95% Conf.
> Interval]
> lapscopic   	.0023183	.0070385	0.33	0.742	-.0114774	.0161141
> snip
> a number of covariates
> snip
> _cons	.7079673	.0127527	55.52	0.000	.6829717	.7329628
>
> How can I revert the result of the linear regresion of ln-transformed
> LOS to difference between laparoscopic and open in days? Exp(0.002)
> gives 1.002 but this can not represent the difference between the
> laparoscopic and open surgical methods.
>
> Somewhere on the statalist I have read that poissonregression can be
> used in this situation. This is the result of a poissonregression:
>
> LOS          	Coef.  	Std. Err.	z	P>z	        [95% Conf.	Interval]
> lapscopic	        -.0225546	.0075029	-3.01	0.003	-.03726	-.0078492
> snip
> a number of covariates
> snip
> _cons	.986855	.0131518	75.04	0.000	.9610779	1.012632
>
> How do I interepret this result? Is the laparoscopic LOS
> significantly
> shorter with 0.02 days?
>
> I would appreciate your help.
>
> Regards
> *
> *   For searches and help try:
> *   http://www.stata.com/help.cgi?search
> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/
>

-----------------------------------------
Maarten L. Buis
Department of Social Research Methodology
Vrije Universiteit Amsterdam
Boelelaan 1081
1081 HV Amsterdam
The Netherlands

Buitenveldertselaan 3 (Metropolitan), room N515

+31 20 5986715

http://home.fsw.vu.nl/m.buis/
-----------------------------------------

*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/