Question

What is the difference between main effect and interaction effect? Give 2 examples, and show why.

Answer 1

We know that yield of the rice is affected by the different level of urea and potassium. For example, we used two levels for each urea and potassium independent variables that are 1 tone per hector and 2 tonnes per hector.

To determine whether the urea and potassium affect the yield

Effect #1: Urea level - Are there any differences in the final yield of rice that can be attributed to the urea levels?

Effect #2: Potassium level - Are there any differences in the final yield of rice that can be attributed to the pottasium levels?

Effect #3: Interaction between urea levels and potassium levels - Is there an interaction between urea levels and potassium levels in determining the final yield of rice?

Here, first and second effects are called main effect and third effect is called the interaction effect. In simple, to test the significance of different levels of the individual independent variable is called the main effect and the interaction of levels of these variables is called the interaction effect. In interaction effect test of this, we check whether there are any significant differences between the different levels combination (1,1) (1,2), (2,1) and (2,2) (first one for urea and the second one for potassium) of these two independent variables.

Example 2.

To determine if drug dosage or original tumor size affect the final tumor size, Jamal needs to test for three different effects.

Effect #1: Drug dosage - Are there any differences in the final tumor size that can be attributed to the drug dosage?

Effect #2: Original tumor size - Are there any differences in the final tumor size that can be attributed to the original tumor size?

Effect #3: Interaction between drug dosage and original tumor size - Is there an interaction between drug dosage and the original tumor size in determining the final tumor size?