A proposed data-driven software quality improvement method has three elements. First, the downstream Customer Quality Metric (CQM) quantifies quality as customers perceive it. On the basis of data collected after systems are deployed, it measures how serious defects affect customers. Second, the upstream Implementation Quality Index (IQI) measures the effectiveness of error removal during development. IQI predicts future customer quality; it has a positive correlation with CQM. Finally, prioritization tools and techniques help focus limited development resources on the riskiest files in the code. This research is based on a multiyear program to improve the quality of delivered systems at Avaya, a global provider of business communication and collaboration systems. Regular reviews with Avaya's R&D Quality Council provided governance for the program.