6.4. Criminologists are interested in the effect of punishment regimes on crime rates. This has been studied using aggregate data on 47 states of the USA for 1960, available in data frame UScrime (Ehrlich, 1973; Vandaele, 1978; Raftery, 1995). The response variable is the rate of crimes in a particular category per head of population. There are 15 explanatory variables; most of these and the response variable have been rescaled to convenient numbers.

(a) Analyse these data. In your report pay particular attention to how your model was selected.

(b) Comment on the effect of the last two explanatory variables in relation to the criminologists' interest in the effect of punishment.

(c) Comment critically on the assumptions needed to draw conclusions from aggregate studies such as this.