Love,

Your professor's advice was a bit vague. SPSS lets you add or subtract

variables stepwise from your equation, deciding on the basis of the impact

of each inclusion or exclusion on the overall fit of the equation. There are

two ways: forward stepwise inclusion and backwards elimination. There is a

PIN criterion for inclusion of a new variable in the equation, and a POUT

criterion for exclusion of a variable already in the equation, the default

values of which are 0.05 and 0.10 respectively, but you may change them at

will. You can find explanations of their meaning in standard books on

logistic regression, such as Hosmer and Lemeshow's Applied Logistic

Regression, and in SPSS documentation such as the Syntax Reference manual

and the procedure Algorithms (both in your installation CD) or Marija

Norusis book on advanced procedures.

On the other hand, besides being vague, the advice was a bit contradictory.

That a variable is non-significant usually means that its impact is so small

that you cannot reliably tell whether it is different from zero. The minimum

effect size for a variable effect to be significant depends, of course, on

sample size and the level of significance you choose. So a 10% impact may be

too much or too little, depending on circumstances. But in the case of

logistic regression a 10% impact means a 10% impact on log odds, and that is

in most circumstances a pretty significant impact, so the variable causing

it is most probably significant. If it is significant (even if the effect is

below 10%) you must leave it in the equation. If it is not, you cannot tell

whether the effect is for real or just a fluke (even if above 10%, which may

happen if your sample is too small or your significance requirements are too

high).

Luv.

Hector

-----Mensaje original-----

De: SPSSX(r) Discussion [mailto:

[hidden email]] En nombre de Love

(sent by Nabble.com)

Enviado el: Tuesday, August 01, 2006 11:57 PM

Para:

[hidden email]
Asunto: Insignificant variable removed in logistic regression

Hi guys,

Can you please tell me which technique i use to measure if a non-significant

variable changed the other variables in the model by 10% when removed. This

is in a logistic regression. A professor told me once that if you remove a

non significant variable from a model, it shouldn't change the other

variables by more than 10%. How can I test this?

Many thanks, Love.

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