1a) What is Marginal Return on Marketing Investment (Marginal ROMI) and why is it preferable in most situations to using Total Return on Investment (Total ROMI)?

1b) What is Ad stock (describe and show Adstock included in a sales prediction model for two types of media)?

1c) What is the term 'VIF value,' (explain how it is used and how to interpret VIF for a variable in a model, provide an example indicating the threshold level(s) you would look for in model evaluation)?

1d) What is a normative model? How does it differ from a predictive model?