2-way anova: SS for parameters not adding to model SS - Why?

deer_dog

Hello

I am helping a student use a traditional two-way ANOVA with JMP 7. We are using 'fit model' and two categorical variables and their interaction. I am using the "Analysis of variance" output for the model SS, the error SS and the total SS. I am using the "effect tests" to get SS and F and p for each variable. (Is this correct so far?).

The problem is that the SS for the parameters is not adding up to the "model" SS. It is pretty close, but not exact. Is a small discrepancy to be expected? Do people typically report SS for parameters these days (I usually use general linear models than traditional ANOVA's so feel out of touch on this one).

Thanks in advance for any assistance!

ron_horne

hey deer&dog,

putting it in simple terms, the different sum of squares you mention are not of the same "Type". for a detailed explenation look up Type 1 sum of squares and type 3 sum of squares.

sticking to a simple explanation, the SS in the effect test table represents the "clean contribution" of that effect to the model. in order to evalue it from the model SS you would need to run the mode with and without the specific effect. in the two different models estimated you can compare the SS of the "full" model (with the effect) and of the "base" model (without the effect) and notice that the difference in SS (of the whole model) is exactly equivalent to the SS of the effect.

hope this gives you a first clue.

ron