st: left censoring, survival analysis

["V. Martini" <vinmar0@hotmail.com>]

Dear Stata Users,

I'm actually working on a sample of employed workers and I would like  
to know how to deal with left censoring in my data.

My sample contains employed workers in 1998, therefore I observe one  
spell per worker.

The end of the spells is found by using following waves.

The beginning of each employment spell can be found because workers  
have been asked when they started the current job.
But unfortunately, this information is not known for workers that  
started their spell before 1979.
My interpretation is that these workers should be treated as left  
censored, as the date when their spell begins is not known. I only  
know that their spell started before 1979.

My questions are:

1) Can I deal with these data in STATA or should I remove the left  
censored observations?
    In my sample, it seems that  having left censoring and duration  
of the spell are positively correlated, therefore deleting these  
observations is likely to have consequences on inference.

2) Is the use of the module INTCENS (by Jamie Griffin) appropriate for  
these data?

3) The guide to Survival Analysis by Cleves, Gould and Gutierrez  
suggests that, even if possibly different in nature, matematically,  
left censored data can be treated as interval censored. In my case, I  
would observe one
interval for each worker (intervals are very long and transitions  
occur at the end of the interval).
Therefore, can I estimate my model using a probit / logit / cloglog  
model?

4) Finally, would st setting my data only indicating right censoring  
invalidate non parametric analysis (specifically, KM and NA estimates)?

Thanks,

Vinicio
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