Until About 10 Years Ago, Most Studies in Labor Economics

Essay

Until about 10 years ago, most studies in labor economics found a small but significant negative relationship between minimum wages and employment for teenagers. Two labor economists challenged this perceived wisdom with a publication in 1992 by comparing employment changes of fast-food restaurants in Texas, before and after a federal minimum wage increase.
(a)Explain how you would obtain external validity in this field of study.
(b)List the various threats to external validity and suggest how to address them in this case.