A Think Tank has hired you to help them interpret some of their estimation results. They are interested in estimating the causal effect of marketing spending (e.g. on advertisements) on firm annual sales. They have assembled a database of a respresentative sample of 1000 firms. They observe each firm for 10 years of data, with a total of 1000 10 = 10,000 observations in the database. They have estimated the following regression specification: = 0 + 1 + 2 + 3 + where: is the natural logarithm of firm annual sales in USD for firm in year . is the natural logarithm of annual marketing spending in USD of firm in year . is a dummy variable that takes the value of 1 if the firm has more than the median number of employees at the beginning of the sample period, and 0 if the firm has less employees than that at the beginning of the sample. This dummy variable does not vary over time within each firm. is a dummy variable that takes the value of 1 if there was a recession in year , and 0 otherwise. This dummy variable does not vary across firms within a