The text discusses Card and Krueger's study on estimating the effect of an increase in the minimum wage and demonstrates how the authors estimate a difference-in-differences effect of an additional 2.7 workers being hired per establishment in Pennsylvania, which was the state that increased its minimum wage. (This article is listed in the Selected Readings section of the text and is available at www.jstor.org.) What are some reasons to question the result of an additional 2.7 workers being hired per establishment in Pennsylvania