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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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