This example gives another illustration of what is meant by interaction between two factors.
Click the checkbox Main effect for Ingredient A and Main effect for Ingredient B to show
the 'best' fit of a model with no interaction between the ingredients. From the residuals (red lines),
The model
under-estimates the relief when both ingredients are at the High level.
Click the checkbox Interaction. The best model with an interaction
between the ingredients fits the data much more closely.
The combination of high levels
of the two ingredients has a particularly good effect.
A study was conducted to investigate the effect of a drug compound in providing
relief for hay fever. In the experiment, two active ingredients (A and B)
were each varied at 3 levels in a factorial design with 4 replicates. There
were 36 hay fever sufferers available and they were randomly allocated to
the 9 treatment combinations. The table below shows the hours of relief
that the subjects reported.
|
Ingredient A |
Ingredient B |
Low |
Medium |
High |
Low |
|
|
|
Medium |
|
|
|
High |
|
|
|