Data on 164-6 lactation records of two commercial Frisian herds were collected in the period from 1981 to 1988 on yield of milk re-corded in 90 days (CODY), 305 days (305DY) and total lactation (TY), lactation length (LP), dry period (DP), age at calving (AC), days open (DO) and calving interval (Cl). Two methods of gross comparison (GC) and polynomial regression analysis (PRA) were used to calculate different sets of multiplicative correction factors for age. DO and LP correction factors were constructed using PRA method based on modal class or maximal production. Lists of these factors were presented. Differences in age-correction factors were consistent across 900Y, 305DY and TY. Consistent trends for DO and LP correction factors were also observed where these factors tend to decrease with the increase of DO or LP. The PRA factors of age were generally overestimated relative to GC factors at younger ages while the reverse trend was observed at older ages. Correction factors of PRA s-cerned to be relatively the most successful method in removing the effect of age on milk yield. Factors of PRA based on either of modal class or maximal-production class for DO or LP were closely similar in removing the effect of DO or LP on milk production.