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st: AW: -senspec- interpretation question


From   "Martin Weiss" <martin.weiss1@gmx.de>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: AW: -senspec- interpretation question
Date   Tue, 10 Nov 2009 22:13:37 +0100

<> 

You should look at the column frequencies, i.e. condition on the disease
status, to get the correct picture. I also compare with official -roctab-,
and -senspec- gives the same answers:


*************
clear
vers 9.2
set obs 1000
set seed 12358	
gen byte test=irecode(uniform(),0,0.33,1)
gen disease=_n>500
tab test disease, row
gen method=irecode(uniform(),0,0.5,1)
bysort method: tab test disease, row
tab test disease if method==1, /*row */  col

senspec disease test if method==1, se(sens) spe(spec) fpos(fpos)  /// 
ntneg(ntneg) fneg(fneg) ntpos(ntpos) nfpos(nfpos) nfneg(nfneg)
list test disease ntneg spec nfneg fneg nfpos fpos ntpos sens in 1/10
//compare to official Stata
roctab disease test if method==1, detail table
*************

If ever you are in doubt about the definitions of all these numbers, look at


*************
webuse lbw, clear
logit low age lwt i.race smoke ptl ht ui
estat clas
************

It is all defined in this last table...

HTH
Martin

-----Ursprüngliche Nachricht-----
Von: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] Im Auftrag von Michael
McCulloch
Gesendet: Dienstag, 10. November 2009 21:46
An: Statalist Statalist
Betreff: st: -senspec- interpretation question

Dear Statalist folks,
I'd like to ask advice on interpreting the output of -senspec-. I've  
created the following simple simulation example which allows me to  
compare the -senspec- output to a simple 2x2 table. The following code  
shows that the absolute numbers from -senspec- agrees with each cell  
of my 2x2 table, but the frequency values are different. Am I  
misinterpreting or mis-applying -senspec-?

clear
set obs 1000
set seed 12358	
gen test=1 + int(2*uniform())	
replace test=0 if test==2
replace test=1 in 1/300		
gen disease=0 in 1/500
replace disease=1 in 501/1000
tab test disease, row
gen method=1 + int(2*uniform())	
bysort method: tab test disease, row
tab test disease if method==1, row
senspec disease test if method==1, se(sens) spe(spec) fpos(fpos)  
ntneg(ntneg) fneg(fneg) ntpos(ntpos) nfpos(nfpos) nfneg(nfneg)
* cellA: ntneg sp  	cellB: nfneg fneg  	cellC: nfpos fpos
cellD:  
ntpos se
list test disease ntneg spec nfneg fneg nfpos fpos ntpos sens in 1/10



Thank you kindly.



Michael McCulloch
Pine Street Foundation
124 Pine Street
San Anselmo, CA 94960-2674
tel:	415-407-1357
fax: 	206-338-2391
mm@pinestreetfoundation.org






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