§ Demographic variables: age, sex, race.

§ Personal history: group incompatibility, birth weight, gestational age, Apgar score at birth.

§ Medical history variables: feeding (breast, formula, breast and formula).

§ Physical examination variables: weight, presence or absence of cephalohematoma or broken clavicle.

§ Laboratory variables: transcutaneous bilirubin levels and serum bilirubin.

§ Income: yes/no

§ Length of stay in emergency department.