We are going to be analyzing the causal effect of early education programs-in particular we will look at head start. 2a. This is a map of introduction of head start programs across counties that was shown in Lecture #3. This map was used to motivate Thompson (2014)'s use of a difference in difference approach to study effect of head start. Explain why this suggests a difference in difference approach is valid. Head Start Not Introduced Introduced in 1966 Introduced in 1967 Introduced in 1968 Introduced in 1969 Introduced in 1970 Figure 1 Head Start Introduction by County, 1966-1970 Notes: Data from National Archives and Records Administration. 2b. What the key assumptions are that allow the use of difference in difference? Describe the treatment group and the control group. 2c. The results for the difference in difference approach are shown in the table #3 below. Describe what the study shows about head start exposure's effect on income, unemployment, high school graduation and college attendance