Re: st: Re: ordinal no longer ordinal

 From Maarten buis To statalist@hsphsun2.harvard.edu Subject Re: st: Re: ordinal no longer ordinal Date Sun, 19 Nov 2006 10:19:23 +0000 (GMT)

```--- Marc Campo <mcam54@yahoo.com> wrote:
> I am entering ordinal variables into LR as
> continuous I have 5
> ordinal indep vars all based on frequencies coded:
> 0 - 0x.day
> 1-1-5/day
> 2-6-10x/day
> 3 - 11-15x/day
> 4 - More than 15
>
> Three of them are highly related and I would like to
> combine them. However, just adding them establishes a
> new variable that is not ordinal.  What to do?

Marc:
Adding the variables is equivalent to constraining the parameters of
the three variables to be the same. Call the three variables you wanted
to combine x1 x2 x3 and the dependent variable y. The unconstained
regression would look like:

y = b0 + b1 x1 + b2 x2 + b3 x3 + other vars

If the regression parameters are constrained to be equal, then the
regression equation would look like:

y = b0 + b x1 + b x2 + b x3 + other vars

which can be simplified to:

y = b0 + b (x1 + x2 + x3) + other vars

Constraining the regression parameters to be equal seems a reasonable
way of combining variables that have the same scale, as is the case in
original scale: the effect of one step increase on your ordinal scale
strictly ordinal to begin with: you already assumed that each one step
the distances between the categories by adding them as continuous
variables.

HTH,
Maarten

-----------------------------------------
Maarten L. Buis
Department of Social Research Methodology
Vrije Universiteit Amsterdam
Boelelaan 1081
1081 HV Amsterdam
The Netherlands

Buitenveldertselaan 3 (Metropolitan), room Z434

+31 20 5986715

http://home.fsw.vu.nl/m.buis/
-----------------------------------------

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