“All models are wrong; some models are useful.” This book describes the academia-industry efforts in adopting Six Sigma methodology for building a practical prediction model for functional test defects in system testing phase. The focus is emphasized on the rational behind the research and systematic way of doing it based on Design for Six Sigma. An overview of Six Sigma is provided as quick understanding to the audience about what the methodology really is and why it is selected for this effort. The research also highlights the use of metrics prior to testing in building up the model. Regression analysis is applied for analyzing the metrics, which later becomes the significant factors for predicting functional defects in system testing phase. Verification process on the selected model is shown towards the end of the book together with the control plan for continuously enhancing and strengthening the model.