Interaction between two continuous variables

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*#:where (''b''<sub>2</sub>+ ''b''<sub>3</sub>X<sub>1''i''</sub>) represents the effect of X<sub>2</sub> on Y at specific levels of X<sub>1</sub>
*#:where (''b''<sub>2</sub>+ ''b''<sub>3</sub>X<sub>1''i''</sub>) represents the effect of X<sub>2</sub> on Y at specific levels of X<sub>1</sub>
*#:and ''b<sub>3</sub>'' indicates how much the slope of X<sub>2</sub> changes as X<sub>2</sub> goes up or down one unit.
*#:and ''b<sub>3</sub>'' indicates how much the slope of X<sub>2</sub> changes as X<sub>2</sub> goes up or down one unit.
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*#In the above example, ‘IQ’ and ‘study’ are the variable names in SPSS given to the IQ and time spent studying by the subjects. ‘Subjects’ indicates the variable containing the case numbers, in this case 1-200 as there were 200 subjects in the study.
*#In the above example, ‘IQ’ and ‘study’ are the variable names in SPSS given to the IQ and time spent studying by the subjects. ‘Subjects’ indicates the variable containing the case numbers, in this case 1-200 as there were 200 subjects in the study.
*#Highlight the text, and click run selection. A new SPSS data editor window should be created at the end of which should be 2 new SPSS variables IQ_c and study_c. You should now save this spss file with a new name.
*#Highlight the text, and click run selection. A new SPSS data editor window should be created at the end of which should be 2 new SPSS variables IQ_c and study_c. You should now save this spss file with a new name.
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===► '''Create the interaction term'''===
===► '''Create the interaction term'''===
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*#In our example, multiple IQ_c x study_c
*#In our example, multiple IQ_c x study_c
*#In SPSS this is accomplished using the "compute" command and typing IQ_c * study_c in the open box.
*#In SPSS this is accomplished using the "compute" command and typing IQ_c * study_c in the open box.
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===► '''Conduct Regression'''===
===► '''Conduct Regression'''===
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*How to conduct the regression analysis?
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*#In SPSS, click on "linear regression" and enter the test score variable as the DV.
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*#Enter the new centered variables as the IVs in the regression analysis
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*#Click "next" and enter both centered variables AND the new interaction variable as the IVs.
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*#Run the analysis
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==Plotting the interaction==
==Plotting the interaction==

Revision as of 06:23, 30 October 2006

Statistical programs, like SPSS, do not always have "point-and-click" commands for every possible statistical test. This page is a description of how to test the interaction between two continuous variables. Below, an explanation of interactions is presented, then the three steps to conduct the interaction is described, and examples are given to help in understanding the steps involved.

Contents

What is an interaction?



Three Steps

There are three steps involved to calculate the interaction between two continuous variables.

Center the two continuous variables


Create the interaction term


Conduct Regression





Plotting the interaction

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