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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 * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: RE: NPTREND problem***From:*"Paul O'Brien" <paul.obrien@afe2.org.uk>

**References**:**st: RE: NPTREND problem***From:*"Nick Cox" <n.j.cox@durham.ac.uk>

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