Where the disability for work resulting from an injury is partial, during the disability and beginning on the eighth day thereof, the employer shall pay the injured employee a weekly compensation equal to two-thirds of the difference between his or her average weekly wage before the injury and the average weekly wage which he or she is able to earn thereafter. (Amended 1963, No. 191, § 5; 1965, No. 67, § 4; 1967, No. 122, § 8; 1981, No. 204 (Adj. Sess.), § 8; 1991, No. 264 (Adj. Sess.), § 2.)