Re: st: left censoring in discrete-time duration model

[Steven Samuels <samplerx@earthlink.net>]

On Nov. 28, 2006, at 2:05 PM, Austin Nicholas wrote:

> I would argue that 1992 is a natural "zero" time for Daniel Simon's
> project, since I am reasonably certain no magazines had websites
> before 1992-

As Austin, Stephen Jenkins, and I have suggested, Daniel is limited  
to analyses of firms without web sites in 1996 if he relies solely on  
the web archive. Now I am not sure he can even do that much with safety.

Daniel:  Taking a start date of 1992, as Austin suggests, can you  
identify from other sources which magazines started publication after  
1992 and the dates of publication? If you can, then as Stephen  
stated, then you can use likelihood methods. If you cannot, you have  
problems for which we have no good solution. It appears from your  
description that the web archive gives only the date of first  
appearance of the web-site. If so, what are you going to do about  
magazines which started publication after 1996?  For example, does a  
magazine with first web-site appearance in 2005 have nine years of  
"risk" or did the website and magazine start simultaneously in 2005?   
At the very least you should try to estimate the number of such  
magazines.


Terminology: I disagree with Stephen that the data are "left- 
truncated." I believe that they are primarily left-censored with  
possible left-truncation.  With left-truncated data the very  
existence of some observations is unknown because they take place too  
early.  The classic example is the fetal life table of miscarriages  
which did not record miscarriages that took place prior to the start  
of prenatal care.  In Daniel's archive, the analogous missed  
magazines are those which stopped publishing after 1992 and before  
1996. (Here the very existence of the magazine, not just the web- 
site, is unknown.)  For left-censored data (or for interval-censored  
data, in general), the analyst knows that the event took place  
between two dates. This is the case for the magazines in the archive  
with web sites in 1996, assuming that the dates for start of risk are  
known.


Steven


________________________
Steven Samuels
18 Cantines Island
Saugerties, NY 12477
001-845-246-0774

samplerx@earthlink.net







*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/