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RE: st: left censoring in discrete-time duration model
On 11/16/06, Daniel Simon <email@example.com> 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.
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