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RE: st: RE: FWD: poisson regression on aggrgregated data


From   "Kieran McCaul" <kamccaul@meddent.uwa.edu.au>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: RE: FWD: poisson regression on aggrgregated data
Date   Tue, 10 Feb 2009 12:05:02 +0900

The model can only predict from what you have given it.  If you group
together all the calendar years from 1998 to 2008, then what the model
is predicting is the overall rate for these years combined.  If you are
confident that there is little variation in the rate by calendar year,
then it would be OK to model it this way.  

On the other hand, if there is variation in the rates by calendar year,
then the average rate over all years will not be a good estimate of the
rates by each calendar year.



______________________________________________
Kieran McCaul MPH PhD
WA Centre for Health & Ageing (M573)
University of Western Australia
Level 6, Ainslie House
48 Murray St
Perth 6000
Phone: (08) 9224-2140
Fax: (08) 9224 8009
email: kamccaul@meddent.uwa.edu.au
http://myprofile.cos.com/mccaul 
http://www.researcherid.com/rid/B-8751-2008
______________________________________________
The fact that no one understands you doesn't make you an artist.

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of [Redacted]
Sent: Tuesday, 10 February 2009 10:32 AM
To: statalist@hsphsun2.harvard.edu
Subject: RE: st: RE: FWD: poisson regression on aggrgregated data

 
 
Thank you very much Kieran for your response. I will difinetly read your
paper.
 
Now my question is if the predicted rate in my final poisson model for
example for time_since_start_cat=3 and year_start_cat=2  is 0.076 or 76
per 1000 person years , what will be the predicted rate :
 
for example for year_start=2003 (whitch has been coded as
year_start_cat=2) and year_since_start=3_4 (which has been coded
> as year__since_start_cat=3)
 
Will it be 0.076 person years? If not how can I have the predicted rate
for ungrouped data using my poisson model.
 
 
 
Many thanks again,
Shell



> Subject: RE: st: RE: FWD: poisson regression on aggrgregated data
> Date: Tue, 10 Feb 2009 06:28:51 +0900
> From: kamccaul@meddent.uwa.edu.au
> To: statalist@hsphsun2.harvard.edu
> 
> OK, I see.
> 
> You have the year that treatment started: 1997,1998,1999,...,2008
> which you have coded as:
> year_start_cat = 1 if year_start=1997 and
> year_start_cat = 2 if year_start=1998,1999,...,2008
> 
> You have time since the treatment started (time_since_start):0_1, 1_2,
> 2_3,...,Max
> coded as:
> time_since_start_cat = 1 if time_since start = 0_1 .
> time_since_start_cat = 2 if time_since start = 1_2 
> time_since_start_cat = 3 if time_since start = 2_3,...,Max
> 
> Then you modelled the incidence rates using these covariates and now
you
> want to predict the rates in each of the calendar years that treatment
> started (1997,1998,1999,...,2008) and in each of the years since
> treatment started (0_1, 1_2, 2_3,...,Max).
> 
> Your model is assuming that the rate in the treatment years 1998 to
2008
> is the same. Similarly, it is assuming that after two years of
> follow-up have elapsed, the rate is the same; constant with time since
> treatment started. In fact, it is assuming that this constant rate is
> the same for all treatment years, 1998 to 2008.
> 
> If these assumptions are correct, then you could use the model to
> predict the rates, but you would need to provide some evidence that
they
> are correct.
> 
> If you have sufficient data, you could use -strate- to generate the
> rates for year_start and time_since start to see if your assumptions
are
> essentially correct.
> 
> strate year_start time_since start
> 
> This would give you the annual incidence rates since treatment started
> and you could graph these for each calendar year. I did something
> similar to this in a study of second primary melanoma rates: 
> McCaul et al. The incidence of second primary invasive melanoma in
> Queensland, 1982-2003. Cancer Causes Control 2008; 19(5):451-8.
> 
> Alternately, you could go back to your original data and use
> Kaplan-Meier to generate hazard curves by calendar year of treatment.
> These should be constant three or more years after treatment and they
> should be similar for 1998 through to 2008. 
> 
> ______________________________________________
> Kieran McCaul MPH PhD
> WA Centre for Health & Ageing (M573)
> University of Western Australia
> Level 6, Ainslie House
> 48 Murray St
> Perth 6000
> Phone: (08) 9224-2140
> Fax: (08) 9224 8009
> email: kamccaul@meddent.uwa.edu.au
> http://myprofile.cos.com/mccaul 
> http://www.researcherid.com/rid/B-8751-2008
> ______________________________________________
> The fact that no one understands you doesn't make you an artist.
> 
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu
> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of [Redacted]
> Sent: Monday, 9 February 2009 9:30 PM
> To: statalist@hsphsun2.harvard.edu
> Subject: RE: st: RE: FWD: poisson regression on aggrgregated data
> 
> Dear Kieran
> 
> 
> No I am not modeling mortality rates, I am modeling the incident of
> experiencing an event by patients using these 2 covariates:
> 
> year_start (year of starting treatment): 1997,1998,1999,...,2008
> time_since_start:0_1, 1_2, 2_3,...,Max
> 
> My original data was not in aggregated form, and patients experienced
> the event (1) or not (0). Then I splitted the follow up time in one
year
> periods and coded year_start,1997 as 1 and 1998+ as 2 and time_since
> start ,0_1 as 1, 1_2 as 2 and 2+ as 3. then aggregated data using
these
> coded covariates:
> 
> strate year_start_cat time_since_start_cat
> 
> saved the output, and used poisson on _D (count of events in each
> group)-created by strate-, above 2 coded covariates (year_startcat,
> time_since_startcat) and the exposure that would be created aggregated
> follow up time _Y ,and then predicted number of events and incident
> rates in each group using predict.
> 
> Now I would like to use the coefficients of this fitted poisson model
> and have the predicted incident rates in disaggregated data (for
> year_start and time_since_start before grouping them)
> 
> for example the incident rate for year_start=2003 (whitch has been
> coded as year_startcat=2) and year_since_start=3_4 (which has been
coded
> as year__since_startcat=3) 
> 
> Would it be correct if I use the same predicted incident rate from
> poisson on grouped data (1998+ and 2+) and use it for above example or
> each case in this group or in this case more events than really is
will
> be predicted? 
> 
> Many thanks,
> Shell
> 
> 
>> Subject: st: RE: FWD: poisson regression on aggrgregated data
>> Date: Mon, 9 Feb 2009 04:09:31 +0900
>> From: kamccaul@meddent.uwa.edu.au
>> To: statalist@hsphsun2.harvard.edu
>> 
>> .
>> 
>> It's not clear from this description what you are modelling. Is it
>> mortality rates?
>> Second, when you say that you have used aggregated data, is this data
>> that you have aggregated or was the data already in aggregated form?
>> 
>> 
>> 
>> ______________________________________________
>> Kieran McCaul MPH PhD
>> WA Centre for Health & Ageing (M573)
>> University of Western Australia
>> Level 6, Ainslie House
>> 48 Murray St
>> Perth 6000
>> Phone: (08) 9224-2140
>> Fax: (08) 9224 8009
>> email: kamccaul@meddent.uwa.edu.au
>> http://myprofile.cos.com/mccaul 
>> http://www.researcherid.com/rid/B-8751-2008
>> ______________________________________________
>> The fact that no one understands you doesn't make you an artist.
>> 
>> -----Original Message-----
>> From: owner-statalist@hsphsun2.harvard.edu
>> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Martin
> Weiss
>> Sent: Sunday, 8 February 2009 7:55 PM
>> To: statalist@hsphsun2.harvard.edu
>> Subject: st: FWD: poisson regression on aggrgregated data
>> 
>> <>
>> 
>> Dear [Redacted],
>> 
>> short answer: I do not know! BTW, it is not clear to me why you want
> to
>> tap 
>> only my wisdom when you can have the entire list. So I am forwarding
>> this 
>> topic to the list for you...
>> 
>> 
>> HTH
>> Martin
>> _______________________
>> 
>> 
>> Dear Martin
>> 
>> I would like to ask the following question.
>> 
>> I have used poisson regression on aggrgregated data with 2 covariates
>> (in a 
>> cohort of HIV positive patients)
>> 
>> 1.year of starting 3+ antiretroviral treatment (1997, 1998+)
>> 2.time since starting 3+antiretrovirals (0-1, 1-2, 2+)
>> 
>> and I have predicted incident rates in this aggregated data using
>> predict 
>> option after the poisson model.
>> 
>> Now my question is if I want to have predicted incident rates for
>> patients 
>> starting 3+ arvs at 2000 and time since starting 3+ arvs , 3_4 years
>> is it correct to use the same predicted incident rate obtained for 
>> aggregated data and in this example use predicted incident rate for
>> 1998+ 
>> and 2+?
>> 
>> It would be greatly appreciated if you could answer my question,
> Please
>> let 
>> me know if you would like me to provide more details.
>> 
>> Thank you very much in advance,
>> 
>> Kind regards,
>> Shell
>> 
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