Suppose that between 2003 and 2007, one group of individuals (Group 1) received job training while another otherwise similar group (Group 2) did not. Suppose that the average earnings of the first group went from $40,000 per year to $55,000 per year, while the average earnings of Group 2 went from $40,000 per year to $50,000 per year. What is the difference-in-difference estimator of the effect of the job training on earnings?