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st: RE: NPTREND problem


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: NPTREND problem
Date   Mon, 11 Nov 2002 17:15:04 -0000

I replied to Paul O'Brien
> > 
> > I have not been able to get the table layout right to do a 
> > test for trend
> > (NPTREND) with the following data:
> > 
> > WEIGHT   EE20mcg EE30mcg
> > gained     48     24
> > stable    210    114
> > lost       38     18
> > 
> > What I get is this:
> > 
> > . tabi 48 24\210 114\38 18
> > 
> >            |          col
> >        row |         1          2 |     Total
> > -----------+----------------------+----------
> >          1 |        48         24 |        72
> >          2 |       210        114 |       324
> >          3 |        38         18 |        56
> > -----------+----------------------+----------
> >      Total |       296        156 |       452
> > 
> >           Pearson chi2(2) =   0.2483   Pr = 0.883
> > 
> > . list
> > 
> >           row       col           pop
> >   1.        1         1            48
> >   2.        1         2            24
> >   3.        2         1           210
> >   4.        2         2           114
> >   5.        3         1            38
> >   6.        3         2            18
> > 
> > . nptrend row, by(col)
> > 
> >        col     score       obs      sum of ranks
> >          1         1         3          10.5
> >          2         2         3          10.5
> > 
> >           z  =  0.00
> >   Prob > |z| =  1.00
> > 
> > 
> > 
> > The correct answer is:
> > One sided P = .2965
> > Two sided P = .593
> > 
> > Where am I going wrong?
> 
> The problem is that Stata does not 
> know that it should look at your -pop- variable. 
> 
> Try 
> 
> . nptrend pop, by(row) 
> 
> Incidentally, a graph created with 
> 
> . ordplot row [w=pop] , by(col) 
> 
> supports the idea that these groups 
> of responses are similar. 
> 
> (For -ordplot-, type -ssc desc ordplot-.) 
> 

No, I think my advice here on -nptrend- 
is wrong: I followed your previous
path too readily. In fact, it 
is not at all clear that this 
is a problem for -nptrend- at 
all. Your response appears 
to be (ordered) categorical 
and your covariate something 
on two levels. 

I think the graph still makes sense
and it suggest that there is no 
relationship worth talking about. 

Nick 
n.j.cox@durham.ac.uk 
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