Re: Fwd: Compare years

Posted by Bruce Weaver on Mar 18, 2013; 7:02pm
URL: http://spssx-discussion.165.s1.nabble.com/Compare-years-tp5718788p5718818.html

Thanks for clarifying.  Here are a few points to consider.  

"Just to compare two snapshots, point prevalences."

1. Given that you have a mix of people with data for both years and others with data for only one of the years, I don't think there is any straightforward way, unproblematic to make the comparison with a single test.  See point 3, however.

2. Prevalence is a proportion, so it's not clear to me why you want to compare odds ratios.  For patients with data for both years, why not just use the McNemar chi-square?  For the others with data for only one of the years, use Pearson's chi-square.  

3. The McNemar and Pearson chi-squares should be independent of each other.  The sum of independent chi-squares is also a chi-square with df = the sum of the df values for each of the independent chi-squares.  In this case, the McNemar and Pearson chi-squares each have df=1, so the sum has df=2.

4. Before trying to publish anything using this method, I'd suggest you wait to hear what other list members have to say about it -- or even what I have to say about it after I've thought about it a little more!  ;-)

5. Given that you are likely dealing with huge numbers of patients, isn't the more important question whether the difference is practically important?  (With huge samples, even tiny, inconsequential differences will be statistically significant.)

HTH.

Øivind Hundal wrote

Yes, all patients have unique identifiers.

Øivind

---------- Forwarded message ----------
From: Bruce Weaver <[hidden email]>
Date: 2013/3/18
Subject: Re: Compare years
To: [hidden email]


Do you have unique identifiers for individuals so that you can match in
cases
where people have data for both years?



Øivind Hundal wrote
 > Hello to you all
>
> I calculate odds ratio for concurrent use of antidepressants when you also
> receive antidiabetic medication. Utilizing logistic regression. How can I
> compare and test difference between 2 years in these ORs? Appreciate
> syntax.
>
> E.g.: OR for concurrent use of drug A when you use drug B in 2005 is X,
> and
> OR for concurrent use of drug A when you use drug B in 2012 is X.
>
> How can I test if OR(2005) is significantly different from OR(2012)?
>
> In both cases we use data from the complete Norwegian population. Our ORs
> are a "snapshot" from each year, and as some patients use drug B in both
> years, there is a partial overlap between the patient groups in these two
> years.
> However, I am not interested in any development from 2005 to 2012. Just to
> compare two snapshots, point prevalences.
>
> Øivind





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Bruce Weaver
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