Summary

By specifying an interaction between the covariate and factor, you are able to test the homogeneity of the covariate parameter estimates across levels of the factor. Since the interaction term was not significant, indicating the covariate parameter estimates are homogenous, you proceeded with an analysis of covariance and found that participation in the program increases salary by $4,357, on average.

If the interaction term was significant, you could use the model with the interaction term, with the understanding that assessing the effect of program participation is complicated by the presence of the interaction. That is, when this interaction term is significant, the difference between participation and nonparticipation changes for different values of Income before the program.

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