Terms of art a Cross sectional vs, time series variation (history consumption function studies Heterogeneity a group effects(individual effects) Fixed effects and or random effects Substantive differences? Is it possible to tell them apart in observed data?Terms of Art  Cross sectional vs. time series variation - (history: consumption function studies)  Heterogeneity  Group effects (individual effects)  Fixed effects and/or random effects ◼ Substantive differences? ◼ Is it possible to tell them apart in observed data?