x1: per capita crime rate by town
x2: proportion of residential land zoned for lots over 25,000 sq.ft.
x3: proportion of non-retail business acres per town
x4: Charles River dummy variable (= 1 if tract bounds river; 0 otherwise)
x5: nitric oxides concentration (parts per 10 million)
x6: average number of rooms per dwelling
x7: proportion of owner-occupied units built prior to 1940
x8: weighted distances to five Boston employment centres
x9: index of accessibility to radial highways
x10: full-value property-tax rate per $10,000
x11: pupil-teacher ratio by town
x12: proportion of blacks by town
x13: % lower status of the population
y: log medv
Median value of owner-occupied homes in $1000's
random split of data in 4 parts