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Friday

I am running a Poisson model in sas comparing the number of ER visits after a surgery between 2 diseases in a given year. For those patients who were diagnosed later on in the year however, we have less follow up time to capture the ER visit so I decided to offset by the month of the surgery. I am wondering if someone may help in interpreting the estimates produced in the model with this offset term.

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Posted in reply to Melk

Friday

Interpret what estimates? From what model? On what data?

PG

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## Re: Offset in poisson model

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Posted in reply to PGStats

Friday

The parameter estimates from the model described above? Did I leave out anything important?

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Posted in reply to Melk

Friday

Yes, the SAS code and the data.

PG

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Posted in reply to PGStats

Friday

proc genmod data=dat;

class disease;

model er_visits = disease / dist = poisson

link = log

offset = logindexmonth;

run;

disease = a or b, er_visits is a count, and logindexmonth is the log of the number of months into the calendar year when the patient had their surgery. alternatively, I think I could also do the number of months left in left in the calendar year to reflect the follow up time but I was not sure which one would be more appropriate.

class disease;

model er_visits = disease / dist = poisson

link = log

offset = logindexmonth;

run;

disease = a or b, er_visits is a count, and logindexmonth is the log of the number of months into the calendar year when the patient had their surgery. alternatively, I think I could also do the number of months left in left in the calendar year to reflect the follow up time but I was not sure which one would be more appropriate.

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Posted in reply to Melk

Friday

You should be using the log of the number of months during which the visits were counted for that patient as an offset.

PG

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Posted in reply to PGStats

Friday

Thank you! So I have an estimate statement for disease just looking at disease 1 -1, for disease a b. With the offset, I am not sure how to interpret the estimate. Does the offset change this?

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Posted in reply to Melk

Saturday

The time unit that you choose for your offset calculation will determine the units of your estimates. For example, a Poisson process with mean 10 per month, during a varying number of months :

```
data test;
call streaminit(7);
do id = 1 to 20;
mth = rand("integer", 12);
logmth = log(mth);
logyr = log(mth/12);
n = rand("Poisson", mth*10);
output;
end;
run;
title "Time expressed in months";
proc genmod data=test;
model n = / dist=Poisson offset=logmth;
run;
title "Time expressed in years";
proc genmod data=test;
model n = / dist=Poisson offset=logyr;
run;
```

Notice how the exponentiated mean estimate is multiplied by 12 when offset is expressed in years instead of months..

PG