# st: RE: NPTREND problem

 From "Nick Cox" <[email protected]> To <[email protected]> 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
to be (ordered) categorical
on two levels.

I think the graph still makes sense
and it suggest that there is no