|
|  |
[Date Prev][Date Next][Thread Prev][Thread Next][Date index][Thread index]
Re: st: left censoring in discrete-time duration model
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
[email protected]
*
* 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/
