I have measured Copper content in fishing nets. I have 2 independent variables -
treatment of the net with 4 levels and
type of net with 2 levels. I'm using R to do the ANOVA.
I care about the interaction, so I should do:
mCu = aov(Cu ~ Type * Treatment, data = Ultrasonic)
But why do I get different values for the difference in type if I do:
mCu = aov(Cu ~ Type + Treatment, data = Ultrasonic)
I thought using the asterisk instead of the plus sign would just add the interactions as well, not change the analysis of the variables by themselves. (Although, the values are not very far off.)