Conceptual Overview: Explore how the coefficients in an interaction equation affect the graphical display. When interaction between two variables is present, we cannot study the effects of one variable (1) on the response y independently of the other variable (2). In other words, meaningful conclusions can be developed only if we consider the joint effect that both variables have on the response; in this case, 12.

The graph depicts the regression lines for two groups (defined by g = 0 and g = 1). If the lines are parallel, then there is no interaction. If the groups have different slopes, then there is an interaction. Move the sliders for the coefficients at the bottom and observe how the graph changes.

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0
2
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-20
-10
0
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20
y = b₀ + b₁x₁ + b₂x₂ + b₃x₁x₂
y = 0 + 0x₁ + 1x₂ + 0.5x₁x₂
g = 0
g = 1
-5
0
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b₀ = 0

-2
0
2
b₁ = 0

-2
0
2
b₂ = 1

-2
0
2
b₃ = 0.5
Created by Gary H. McClelland, Professor Emeritus | University of Colorado Boulder

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