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

 From [Redacted] To Subject RE: st: RE: FWD: poisson regression on aggrgregated data Date Tue, 10 Feb 2009 01:32:28 +0000

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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,
>> 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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