The store manager's analytics team has observed that unit sales in a given week vary depending upon the month, on whether or not a promotion has occurred in the current week, and whether a promotion has occurred in the prior week. Using historical data, the retailer has regressedunit weekly salesonmonthly dummy variables (with June as the base case), adummy variable for whether there was a promotion that week or not(coded 0 for no promotion and 1 for a promotion) and adummy variable for a one week lag after the promotion. The regression coefficients from the analysis is reproduced below.

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