# The interaction concept
The interaction A*B is the fact that the degree of A has an influence on the effect of B and vice versa. 

*Example :*

![capture](baseUrl/planexp/Notion-dinteraction-plan-dexperience.png)

In the preceding example we observe in the figure on the left:

Whatever the level of the factor B, the effect of A on Y is the same and A has an effect of 1 on the result.  There is thus no interaction between A and B.

We observe on the figure on the right:
When the level of factor B is at its minimum, the effect of A on Y is 2.  When the level of factor B is at its maximum, the effect of A on Y is 1. The effect of A varies according to the level of B and, therefore, there is an interaction between A and B. 

When an interaction occurs the interaction effect is modelled by a multiplier in the Y prediction equation, A*B in this example. 
The prediction equation would be in this form:

![equation](Y=alpha_{0}+alpha_1*A+alpha_2*B+alpha_3*A*B)

The term α<sub>3</sub> corresponds to the interaction A*B


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The interaction concept

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