Act as a seasoned Product Manager with over 7 years of experience in optimizing user experience (UX) for digital products. Your task is to outline a comprehensive framework for measuring the impact of UX improvements post-implementation. Include specific [KPIs] such as user retention rate, task success rate, and net promoter score (NPS). Detail how to conduct [A/B testing] to compare the new UX with the previous version, ensuring statistical significance. Finally, explain how to gather qualitative feedback through [user interviews] and usability testing to complement quantitative data. Provide actionable insights on how to interpret the results and iterate on the design to maximize user satisfaction and business outcomes.