13.23. For the "2005 statewide crime" data file at the text Web site let z be a dummy variable for whether a state is in the South, with z = 1 for AL, AR, FL, GA, KY, LA, MD. MS, NC, OK, SC, TN, TX, VA, WV.

(a) Not including the observation for D.C., analyze the relationship between y = violent crime rate and z, both ignoring and controlling for x - poverty rate. Summarize your analyses and results.

(b) Repeat the analysis with D.C. in the data set, setting z = I for it. Is this observation influential?