3. (Interpretation of Simple Linear Regression Model) Suppose we collect data on 230 mothers' number of cigarettes they have per day during pregnancy and the birth weight of their child (in grams). Suppose now that you run a regression of the birth weight (W.) on the number of cigarettes (C.) and find that W. = 3396 15C, + .. (a) Thinking about the model in a descriptive manner. Explain what it means that the slope coefficient is -15. (b) Thinking about the model in a causal manner. Explain what it means that the slope coefficient is -15. (c) Do you think the causal interpretation is valid?