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
--- roland andersson <[email protected]> wrote:
> 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
> Roland Andersson
> *
> * 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
visiting address:
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/
