I would like to see the effect of two independent variable (a,b) on a dependant variable (c). a and b are covariate.

Could you please let me know how to do two-way anova if

1) there is an interaction between a and b

2) there is no interaction between a and b

I am quite confused with this.

I've done the two-way anova with and without including the interaction term (a*b), the results are totally different.

When I include the interaction term, a and b significantly affect c (both p values are less than 0.01), but when I don't include the interaction term, a affects c but b doesn't (p value greater than 0.01).

Thank you,

Aj