Empirical and factorial are two ways to determine nutritional requirements for animals. The three purposes of this study, done by L. Hauschild et al, were to estimate nutrient requirements for a pig population using empirical and factorial methods, establish a relationship between the requirements that relate the two methods, and to study the limitations of these methods when used to determine the nutrient requirements for individual pigs and groups of pigs. Empirical and factorial methods were used to estimate the ratio of lysine to Net energy (Lys:NE). Optimum ratios were then determined using a systematic analysis. Sixty-eight individual pigs were then simulated using experimental data. For the empirical method,  population responses were recorded by using 11 different Lys:NE diets. Average daily gain and feed conversion ratio were used as the performance criteria. Optimal Lys:NE requirements for growing pigs were found to be different depending on the method used. The factorial method does not allow estimates of Lys:NE that maximize the response of similar populations in a given time and weight interval. The empirical method was able to attain optimal population responses, although the estimation requirements are fixed and cannot be used for a population or a growing phase. This study shows that the two common  methods to calculate nutrient requirments have different results, and important limitations that must be considered when the goal is to optimize response in a individual or group of pigs.