What is an Interaction?

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*Interactions are when the effect of two, or more, variables is not simply additive. This page describes the interaction between two variables. It is possible to examine the interactions of three or more variables but this is beyond the scope of this page.
*Interactions are when the effect of two, or more, variables is not simply additive. This page describes the interaction between two variables. It is possible to examine the interactions of three or more variables but this is beyond the scope of this page.
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==Examples==
 
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*Imagine a study about the effect of energy bars and energy drinks on time to run the 1500 meters. The quantity of energy bars and energy drinks represent two variables. The dependent variable is the time taken to run 1500 meters.
 
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*# ''Example 1'' - An interaction occurs if running speed improves by more than just the additive effect of having either an energy bar or an energy drink. For example, imagine eating a certain amount of energy bars increases running speed by 5 seconds, and drinking energy drinks increases running speed by 3 seconds. An interaction occurs if the joint effect of energy bars and energy drinks '''increases running speed''' by more than 8 seconds, such as liquid in the drink amplifying the ability to digest the energy in the bar leading to faster times.
 
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*# ''Example 2'' - Another example of an interaction effect would be if the joint effect of energy bars and energy drinks '''decreases running speed''' -- perhaps the person feels bloated from eating ''and'' drinking and so are unable to run quickly, or perhaps the chemicals in the energy bar and energy drink counteract each other.
 
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*# ''Example 3'' - A third example of an interaction is that alone neither variable may have an effect on running speed, such as imagining that an energy bar by itself, or an energy drink by itself, is unable to increase running speed. But, there might be an interaction effect that influences running speed when you eat the bar ''and'' drink the drink, such as the energy bar having a chemical that unleashes the power of the energy drink to increase running speed.
 
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__TOC__
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==Interactions and Main Effects==
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*Imagine a study about the effect of energy bars and energy drinks on time to run the 1500 meters. The quantity of energy bars and energy drinks represent two variables. The dependent variable is the time taken to run 1500 meters.
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*# ''Example 1'' - An interaction occurs if running speed improves by more than just the additive effect of having either an energy bar or an energy drink. For example, imagine eating a certain amount of energy bars increases running speed by 3 seconds, and drinking energy drinks increases running speed by 2 seconds. An interaction occurs if the joint effect of energy bars and energy drinks increases running speed by more than 5 seconds, such as liquid in the drink amplifying the ability to digest the energy in the bar leading to faster times.
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*#:Chart 1a below shows an additive effect
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*#:Chart 1b below shows an Interaction.*
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*# ''Example 2'' - A second example of an interaction is that alone neither variable may have an effect on running speed, such as imagining that an energy bar by itself, or an energy drink by itself, is unable to increase running speed. But, there might be an interaction effect that influences running speed when you eat the bar ''and'' drink the drink, such as the energy bar having a chemical that ''unleashes'' the power of the energy drink to increase running speed.
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*#:Chart 2a shows when neither variable has an effect, with no Interaction
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*#:Chart 2b also shows when neither variable has an effect, but now with an Interaction
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*# ''Example 3'' - A final example is when one of the variables has an effect but not the other. When a variable has an effect (such as the energy bar increasing running speed, or the energy drink increasing running speed) that is called a Main Effect.
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*#:Chart 3a shows a Main Effect for the energy bar, with no Interaction
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*#:Chart 3b shows the same Main Effect for the energy bar, but now with an Interaction
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*#:Chart 4a shows a Main Effect for the energy drink, with no Interaction
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*#:Chart 4b shows the same Main Effect for the energy drink, but now with an Interaction
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==Graphical representations of interactions==
 
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[[Image:2ME.jpg|alt text]]
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<nowiki>*</nowiki>(FYI - Chart 1b shows an ordinal interaction. A disordinal interaction occurs when the two lines cross).
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==Graphical representations of Interactions==
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<table width=100% cellpadding=20>
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<td width=50% >
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This column shows the four possibilities WITHOUT an interaction:
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#Two Main Effects
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#No Main Effects
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#Main Effect for Variable A
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#Main Effect for Variable B
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</td>
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<td width=50%  valign=top>
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This column shows the four possibilities WITH an interaction:
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#Two Main Effects
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#No Main Effects
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#Main Effect for Variable A
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#Main Effect for Variable B
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</td>
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</table>
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<table>
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<td>
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There are other graphical representations of Interactions and Main Effects such as effects being reduced (e.g., imagine all the graphs below being flipped down) or effects going in the opposite direction (e.g., imagine all the graphs being flipped to the left), but for simplicity sake they are not displayed because you can discern those graphs after identifying the major types described below.
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</td>
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</table>
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<table>
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<td>
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[[Image:NoI2MEc.JPG]]
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</td>
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<td>
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[[Image:I2ME.JPG]]
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</td>
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</table>
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<table>
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<td>
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[[Image:NoINoME.JPG]]
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</td>
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<td>
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[[Image:INoME.JPG]]
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</td>
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</table>
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<table>
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<td>
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[[Image:NoIMEb.JPG]]
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</td>
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<td>
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[[Image:IMEb.JPG]]
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</td>
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</table>
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<table>
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<td>
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[[Image:NoIMEd.JPG]]
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</td>
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<td>
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[[Image:IMEd.JPG]]
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</td>
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</table>
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==Statistical formula behind interactions==
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==Statistical formula behind Interactions==
*For those more technically minded, here is the algebra. An interaction effect reflects the effect of the interaction controlling for the two predictors themselves.  
*For those more technically minded, here is the algebra. An interaction effect reflects the effect of the interaction controlling for the two predictors themselves.  
*#In the following examples:  
*#In the following examples:  
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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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----
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◄ Back to [[Analyzing Data]] page

Latest revision as of 02:07, 24 November 2010


Contents


Interactions and Main Effects


*(FYI - Chart 1b shows an ordinal interaction. A disordinal interaction occurs when the two lines cross).

Graphical representations of Interactions

This column shows the four possibilities WITHOUT an interaction:

  1. Two Main Effects
  2. No Main Effects
  3. Main Effect for Variable A
  4. Main Effect for Variable B

This column shows the four possibilities WITH an interaction:

  1. Two Main Effects
  2. No Main Effects
  3. Main Effect for Variable A
  4. Main Effect for Variable B

There are other graphical representations of Interactions and Main Effects such as effects being reduced (e.g., imagine all the graphs below being flipped down) or effects going in the opposite direction (e.g., imagine all the graphs being flipped to the left), but for simplicity sake they are not displayed because you can discern those graphs after identifying the major types described below.

NoI2MEc.JPG

I2ME.JPG

NoINoME.JPG

INoME.JPG

NoIMEb.JPG

IMEb.JPG

NoIMEd.JPG

IMEd.JPG



Statistical formula behind Interactions




◄ Back to Analyzing Data page

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