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05-06-2009 04:46 PM

Dear all,

I have checked all the day on the internet without having found anything... that's why I would need your input!

I perform logistic regression. Due to a confounding effect, a non-significant covariate was kept in the model. Following the test of all possible 2*2 interactions between covariates: this variable (initially non-significant) is included in a significant interaction!! And, this variable is now significant.

For example:

- Step 1: selection on covariates

P

Covariate A >0.05

Covariate B >0.05

--> these variables were kept in the model due to their confounding effect

- Step 2:

P

Covariate A <0.001

Covariate B >0.05

Inter A*B <0.001

According to you: what does it mean?

I know that when there is a significant interaction, we cannot interpret the corresponding coefficients independently. But, is it the same for the p-values?

**In particular, could I conclude: "covariate A is significantly associated with Y"???**

Or do you think that I can only say that there is a significant interaction between covariates A and B?

Thanks so much in advance for your answer. I will be a great help!

Best regards,

Violaine

I have checked all the day on the internet without having found anything... that's why I would need your input!

I perform logistic regression. Due to a confounding effect, a non-significant covariate was kept in the model. Following the test of all possible 2*2 interactions between covariates: this variable (initially non-significant) is included in a significant interaction!! And, this variable is now significant.

For example:

- Step 1: selection on covariates

P

Covariate A >0.05

Covariate B >0.05

--> these variables were kept in the model due to their confounding effect

- Step 2:

P

Covariate A <0.001

Covariate B >0.05

Inter A*B <0.001

According to you: what does it mean?

I know that when there is a significant interaction, we cannot interpret the corresponding coefficients independently. But, is it the same for the p-values?

Or do you think that I can only say that there is a significant interaction between covariates A and B?

Thanks so much in advance for your answer. I will be a great help!

Best regards,

Violaine

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05-06-2009 09:03 PM

You are actually going to have to read a book (you know, those dusty things on shelves) for this one. I think the phenomenon is described in Frank Harrell's regression book.

What you saw is why we check interactions first.

All of the p-values are for the variable after considering all the other effects, so you can't say that A is significantly associated with Y. What you can say is that A has a different effect on Y depending on the value of B.

If you look at the coefficients for A in model 1 and model 2, you are likely to see that the coefficient is very different (maybe even changes sign).

Doc Muhlbaier

Duke

What you saw is why we check interactions first.

All of the p-values are for the variable after considering all the other effects, so you can't say that A is significantly associated with Y. What you can say is that A has a different effect on Y depending on the value of B.

If you look at the coefficients for A in model 1 and model 2, you are likely to see that the coefficient is very different (maybe even changes sign).

Doc Muhlbaier

Duke

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05-07-2009 01:55 AM

Thanks so much for your answer! It's the very beginning of the morning in France... so, thanks to you, I will be able to work on my statistical report today! Thanks again! BR, Violaine

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05-08-2009 06:52 AM

doc gave a very good reply.

Here's another take: This is further evidence for NOT using p-values as the basis for much of anything. They rarely, if ever, answer the question you are interested in.

Here's another take: This is further evidence for NOT using p-values as the basis for much of anything. They rarely, if ever, answer the question you are interested in.

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05-08-2009 08:48 AM

plf515: If you were a journal editor, your advice would me more credible. I've had a paper that the editor would not accept until I put p-values on the univariate analyses. Like it our not, p-values are a necessary part of getting published in a journal in most applied fields.

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05-08-2009 12:32 PM

Doc, I know how you feel.

I actually am a statistical reviewer for one journal, and, while I do not (yet) campaign to eliminate p-values, I do insist on measures of effect size.

I actually am a statistical reviewer for one journal, and, while I do not (yet) campaign to eliminate p-values, I do insist on measures of effect size.