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# Re: st: Including components of a summative score in regression

 From Richard Williams <[email protected]> To [email protected], Statalist Statalist <[email protected]> Subject Re: st: Including components of a summative score in regression Date Mon, 30 Jul 2012 16:12:35 -0500

```At 02:46 PM 7/30/2012, Donald Spady wrote:
```
```Dear Statalisters

```
I am doing some logistic regression analysis, some of the variables of which are made up of the values of other variables; e.g. N = A + B + C/D. Is it reasonable, or appropriate, to include A, B, C, or D in the equation if N is already in it.
```i.e.
```
logistic X F G H N A B C D, where F G H are some variables, and N is made up of A B C D, but for some reason or other A B C D are desired to be in the equation.
```
```
My impression is that statistical theory would say this is a no-no, largely because of collinearity; however, if I do it, sometimes I get a better 'fit' to the equation (using estat gof, group(10)).
```
Thanks

```
```
I believe the improvements in fit stem from

```
* N constrains the coefficients of A and B and C/D to be equal; adding A and B relaxes two of those constraints.
```
* The linear effects of C and D are not captured by N

So sure, adding those vars can improve fit.

I could be wrong, but I believe if you ran

logistic X F G H A B C D C/D

```
you would get the same fit as you are getting now. That might seem a little less convoluted to me. But I don't think what you are doing is inherently evil; you just have to understand what the parameters mean and why you get a better fit.
```

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Richard Williams, Notre Dame Dept of Sociology
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```