The Product Activity Coefficient is a metric that measures customer engagement with a SaaS product by tracking the number of weeks a customer has been active over the last 7 weeks, resulting in a normalized score between 0 and 7. This metric helps Customer Success teams prioritize their work by identifying power users, accounts at risk of churning, and opportunities for expansion. By focusing on leading indicators rather than lagging metrics such as churn or customer satisfaction, CS teams can be proactive and help customers before it's too late. The metric is easy to understand and can be computed using product usage data, making it a valuable tool for B2B SaaS companies looking to improve their Customer Success efforts.