Re: Why is 95% CI for ICC so large?

Posted by J McClure on Mar 11, 2011; 2:30am
URL: http://spssx-discussion.165.s1.nabble.com/Why-is-95-CI-for-ICC-so-large-tp3422764p3424470.html

Thanks RIch and David for the help!
Jan

On 3/10/2011 3:28 PM, Rich Ulrich wrote:

> The main difference between a simple ICC and a Pearson R is that
> the ICC *assumes*  that the means are the same.  The result is a
> smaller value for the ICC than for the Pearson when the means differ.
>
> I expect that your ICC for the depressed group has a wider CI
> because of an underlying mean difference, since that is the only
> potential difference I can think of, to account for the discrepancy.
> The N's are actually larger for the depressed group, which would
> tend to give a smaller CI.  Otherwise -- Are you sure that all the
> data were included in the analysis, nothing Missing?
>
> With two raters, I prefer to look at SPSS's paired t-test between raters.
>
> That shows me both the mean difference and the direct correlation.
>
>
>
> --
> Rich Ulrich
>
> ----------------------------------------
>> Date: Wed, 9 Mar 2011 20:53:52 -0800
>> From: [hidden email]
>> Subject: Why is 95% CI for ICC so large?
>> To: [hidden email]
>>
>> Hi,
>> I have run ICC's with a 95% CI to compare agreement self reported rating
>> by the patient and the doctor's rating by another characteristic. For
>> example, I divided the sample between the patient's who had major
>> depression and those without major depression and determined the
>> agreement of the MD's with patients with major depression and separately
>> agreement with patient's without major depression.
>> The ICC for those without depression (n=153) was 0.43 (95%CI 0.23, 0.52
>> ) and for those with depression (n=224) it was 0.43 (CI 0.04, 0.66).
>> Why is the CI for the second group so much wider than for the first? The
>> ICC syntax I am using is /ICC=model (random) type(absolute).
>> Thanks for any help,
>> Jan
>
>
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