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I have a regression with my dependent variable being the number of visitors and my independent variable being the presence or absence of a certain kind of ad.

Now, when I run the regression my independent variable is significant. When instead, I add another independent variable (the presence of a big exhibition at the museum) the same variable becomes non significant.

Can someone enlighten me on what to make of this result?

Thank you very much!

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1 Answer 1

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Welcome to the site.

This means that, after controlling for the effects of the big exhibit, the significance of the ad is lessened. You should also look at the effect sizes (parameter estimates).

It also implies that there is some relationship between the ad and the exhibit. In your case, this makes perfect sense: Quite likely, different sorts of ads are run when there is a big exhibit.

You can examine this relationship by making a crosstabulation of "type of ad" and "presence of a big exhibit".

There may be an interaction effect as well, you can examine this by adding "presence of exhibit"*"Type of ad" to the model.

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  • $\begingroup$ Hi Peter! Thanks for taking time to answer :) I have added the interaction factor to the model with the ad and the exhibition but the interaction is not significant. Maybe I should specify that I am looking at a specific kind of ad, it doesn't change throughout the period analyzed. Does that help? $\endgroup$
    – Lithiar
    Commented Nov 28, 2013 at 12:59

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