External validity. For each of the following scenarios explain why there might be a threat to external validity and suggest a possible solution. a. The Perry preschool program was an expensive randomized controlled trial carried out in a town in Michigan in the 1960s with around 100 program participants that assessed the effect of a high quality preschool program on outcomes for disadvantaged children. Follow-ups have found that this preschool program improved educational outcomes, led to higher earnings as an adult, and reduced the likelihood of interactions with the criminal justice system. Based on this evidence some policymakers in the US have proposed free preschool education at the state or federal level. b. According to one study based on a nationally representative sample of 30 year olds, college graduates earn 50% more than individuals that did not graduate from college. However, this study fails to take into account the fact that women with college degrees only earn 30% more than women that did not graduate from college. c. I use state level data to estimate the effect of the minimum wage on unemployment and I find that each $1 per hour increase in the minimum wage increase the unemployment rate by only 0.2 percentage points. In the data, the minimum wage at the state level ranges from $7 per hours to $12 per hour. For each of the following proposed policies, consider (a) to what extent external validity is a serious problem in using the results of this study to assess the proposed policy and (b) how you could address this, if at all. (Note: Suppose for this problem that the study was internally valid. In reality, internal validity for this particular empirical question is difficult.) External validity. For each of the following scenarios explain why there might be a threat to external validity and suggest a possible solution. a. The Perry preschool program was an expensive randomized controlled trial carried out in a town in Michigan in the 1960s with around 100 program participants that assessed the effect of a high quality preschool program on outcomes for disadvantaged children. Follow-ups have found that this preschool program improved educational outcomes, led to higher earnings as an adult, and reduced the likelihood of interactions with the criminal justice system. Based on this evidence some policymakers in the US have proposed free preschool education at the state or federal level. b. According to one study based on a nationally representative sample of 30 year olds, college graduates earn 50% more than individuals that did not graduate from college. However, this study fails to take into account the fact that women with college degrees only earn 30% more than women that did not graduate from college. c. I use state level data to estimate the effect of the minimum wage on unemployment and I find that each $1 per hour increase in the minimum wage increase the unemployment rate by only 0.2 percentage points. In the data, the minimum wage at the state level ranges from $7 per hours to $12 per hour. For each of the following proposed policies, consider (a) to what extent external validity is a serious problem in using the results of this study to assess the proposed policy and (b) how you could address this, if at all. (Note: Suppose for this problem that the study was internally valid. In reality, internal validity for this particular empirical question is difficult.)