Education reform is one of the most hotly debated subjects on both state and national policy makers' list of socioeconomic topics. Consider a linear regression model that relates school expenditures and family background to student performance in Massachusetts using 224 school districts. The response variable is the mean score on the MCAS (Massachusetts Comprehensive Assessment System) exam given to 10th graders. Four explanatory variables are used: (1) STR is the student-to-teacher ratio, (2) TSAL is the average teacher's salary in $1,000s, (3) INC is the median household income in $1,000s, and (4) SGL is the percentage of single-parent households. A portion of the data is shown in the accompanying table.

SCORE	STR	TSAL	INC	SGL
227.00	19.00	44.01	48.89	4.70
230.67	17.90	40.17	43.91	4.60
230.67	19.20	44.79	47.64	5.10