Date: Fri, 5 Sep 2008 14:38:01 -0400
Reply-To: Richard Ristow <wrristow@mindspring.com>
Sender: "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From: Richard Ristow <wrristow@mindspring.com>
Subject: Re: Modeling Death a Continuous Variable
In-Reply-To: <748bdf0f0808281353s38205023yd15180624ad339ea@mail.gmail.co m>
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To weigh in late, with the advantage of reading other responses --
At 04:53 PM 8/28/2008, Max Gunther wrote:
>We are looking at the effect of a drug that was
>randomized between two groups. We would ideally
>like to operationalize this by using a
>continuous variable that we can call "severity
>of illness." Since many of the patients die
>while they are being studied, and they die at
>higher rates in the drug B group, it ends up
>making *the data look like drug B patients do
>better because there are no "severity of
>illness" ratings since they are deceased.*
>
>*I would ideally like to use death as a
>quantitative end point of the continuous
>variable "severity of illness" but am not sure how to do this.*
Let me restate this, as I understand it:
You have an operational measure "severity of
illness." You are satisfied that it is (or
measures) the underlying quantity of interest,
and that it is of scale level. It is the dependent variable in your analysis.
You wish to make deceased patients available for
analysis, by assigning them a "severity" score.
It looks like (this is less clear),
1. You are comfortable with giving all deceased
patients the same severity score
2. Your severity scale is closed-ended; that is,
there it has an inherent maximum value. You wish
to assign deceased patients some score higher than this maximum value.
3. You want the result to still be a valid scale-level value.
I suggest,
a. Simply assign deceased patients the maximum
severity score. You can then argue that, since
the 'correct' value is probably higher than this,
and group B has higher mortality, your analysis
is conservative in that it has a bias against finding group B inferior.
b. You can cut loose from your severity score and
use mortality as your outcome measure. You should
carry out and publish this analysis, in any case.
c. Finally, just how sure are you, that your
severity measure is scale-level in the first
place? A lot of people would consider ordinal
analysis in your situation; and for that, all you
need do is assign deceased patients some
convenient severity value above the top of the scale.
On SPSSX-L, I think of Marta García-Granero and
Anita van der Kooij as the experts on ordinal
analysis, far beyond my level; there are others,
as well. I don't know if anybody wants to weigh in on the idea.
-Best of good fortune,
Richard
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