Apparent ileal digestibility (AID) of crude protein (CP) and amino acids (AA) is affected by CP and AA concentrations in diets because of the different contributions of endogenous N and AA to the ileal digesta from pigs fed diets with different concentrations of CP. Because of the influence of dietary CP and AA on calculated values for AID, values for AID obtained in individual feed ingredients are not always additive in mixed diets if the concentration of CP and AA in the mixed diet is different from that of the ingredients. Therefore, values for standardized ileal digestibility (SID) of CP and AA, which are corrected for the basal ileal endogenous losses of CP and AA, are used in diet formulations to avoid the influence of endogenous AA on digestibility values, and SID values are, therefore, additive in mixed diets.