I would start back at defining your measurements, and what is the intervention.  The outcome is not a test, the trial intervention is the test.  The outcome should be stated with simple data measures, percents, etc.  The intervention should have the descriptive measures applied, i.e.  PPV, sens., spec., etc.  I hope this is helpful, although my insights may not be complete.

Kind Regards,
Daniel L. Sontheimer, M.D.
Asst. Prof. Of Family Medicine
Spartanburg Family Medicine Residency
853 N. Church Street Suite 510
Spartanburg, SC 29303
Email:  [log in to unmask]

-----Original Message-----
From:   owen dempsey [SMTP:[log in to unmask]]
Sent:   Monday, March 27, 2000 6:23 AM
To:     Evidence-Based-Health
Cc:     Tim Coleman; Richard Neal; Michael. [log in to unmask] wakeha. northy. nhs. uk; Dawood Dassu
Subject:        accuracy of test and confidence in trial outcomes

dear all
a problem is puzzling us
we have a RCT design; to simplify a bit: the primary outcome of interest
is in effect a test (patient recall of being asked about their smoking
by their GP); data suggests a sensitivity of approx 90% and specificity
80%.  The PPV  (which is the necesaary link between the test and the
outcome of interest) will therefore vary depending upon the prevalence
of the target condition (whether a patient was asked about their smoking
at the last consultation with a GP).  The trial intervention is designed
to affect this target condition (to increase the prevalence); therefore
the PPV of the test will vary in a way that we cannot predict. (and
whose only measure is the PPV; which becomes a circular argument))
This seems to introduce another source of error affecting our ability to
be confident about the effect of the intervention.
Is this a significant problem or am I missing something?
Have others had to deal with this; if so how?
presumably this is a common trial problem when the outcome is measured
via a test (less than 100% accurate)of any sort; has anything been
written about this?
TIA; you're my last hope
O
Owen Dempsey
General Practitioner
Senior Research Fellow
Centre for Research in Primary Care
Hallas Wing
Nuffield Institute for Health
71-75 Clarendon Road
University of Leeds
Leeds LS2 9PL
work:  01484-460298  (practice)
work   0113-233-4835 (Leeds office)
home: 01484-654794
e-mail: [log in to unmask] (home)
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