What is an Interaction?

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==Examples==
==Examples==
*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.  
*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. Chart 1 below shows an additive effect, and Chart 2 shows an Interaction.
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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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*# ''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. Both Chart 3 and Chart 4 below show when neither variable has an effect, with Chart 3 also showing no Interaction, and Chart 4 showing the Interaction.
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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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Revision as of 02:38, 29 June 2007

Examples



Graphical representations of interactions

►When an energy bar increases running speed, that is called a "Main Effect" (ME) for the energy bar; and when the energy drink also increases running speed, that is a ME for the energy drink. Chart 1 below shows the graphical result when there are two ME but no Interactions.


File:NoI2MEb.JPG

I2ME.JPG



Statistical formula behind interactions

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