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

[Steven Samuels <samplerx@earthlink.net>]

On 11/16/06, Daniel Simon <dhs29@cornell.edu> wrote: Hi - I'm using - 
hshaz- to estimate a discrete-time hazard model. I have some left  
censoring that I'm not sure how to deal with. I am looking at firms  
establishing websites. I can only observe the introduction of  
websites from 1996 onwards.  However, I know that some firms  
established websites prior to 1996, but I'm not sure which ones.  
Currently, I have tried three approaches: (1) Treat all firms that  
had a website in 1996 as if they adopted in 1996 (the first year of  
the sample period), whether they adopted in 1996 or adopted earlier;  
(2) Exclude 1996 from the sample (begin the analysis with 1997); (3)  
Drop all observations from 1996 for firms that had websites.


Daniel, you do NOT have left censoring OR truncationy, because you do  
not have a natural ZERO starting time.   Your time-dimension is  
calendar time.  Therefore your only analysis choice is number (3).   
However if all you have is the year of startup, then you cannot  
distinguish firms with websites at the start of 1996 and firms whose  
websites appeared first in 1996, so you must choose option (2). If  
you do have calendar dates of web site startup,  use a continuous  
rather than discrete-time model with day or week as a time-unit.

You have to decide how to deal with firms who  began publishing after  
1996.  Many of them will START with web sites, so calendar year of  
start will be a strong predictor of  start of a website.


You can compare characteristics of firms with and without websites in  
1996, but you cannot say anything about the duration of time with web  
site for firms with websites in 1996.

Steven










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