Pile group design is mainly a settlement concern since the stiffness of the pile group is generally smaller than the sum of the individual stiffnesses of each pile: stiffness efficiency is less than 1. The pile-to-pile interaction factor (IF) method could be considered as the most rational and physically acceptable method for calculating pile group settlement compared to other semiempirical methods such as 'equivalent pier' or 'equivalent raft' methods. It superimposes the effects between all possible pairs of piles within a pile group and uses elastic pile-soil-pile interactions. In this paper, old and recently developed closed-form solutions for the IF are reviewed as well as points of attentions that should be considered. It was concluded that the IF approach leads to a huge decrease of the group stiffness even with the consideration of small strain soil stiffness parameters. Furthermore, unlike experimental results, the calculated settlement ratio is independent of the load level. To investigate the effect of the soil nonlinearity on the IF, 3D Finite Element Method (FEM) simulations are performed. It was shown that the IF diminish as pile loading increases. An updated value of the IF in function of the load level provides a better agreement with the measurements.