Flexible Interpretive Structural Modeling (FISM) is an extended and improved version of Warfield's ISM. The algorithm of FISM is based on the Partially Filled Reachability Matrix (PR matrix) model. The PR-matrix model is an extension of a Reachability Matrix (R-matrix) model and has great utility in all phases of ISM. While, or after an Interpretive Structural Model is developed, the stakeholders may want to make amends. The procedures for "editing" an ISM or FISM structural model with the aid of a computer are called Correction Procedures. Correction procedures are the key to FISM.