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Re: st: RE: NPTREND problem


From   "Paul O'Brien" <paul.obrien@afe2.org.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   Re: st: RE: NPTREND problem
Date   Mon, 11 Nov 2002 20:14:44 +0000

Thanks Nick,

The treatments are ethinly estradiol (EE) 20mcg v EE 30 mcg and response is
weight change, so I think your first response was right. Is that so?

Paul

On 11/11/02 5:15 pm, "Nick Cox" <n.j.cox@durham.ac.uk> wrote:

> 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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