You are a product growth analyst for a consumer tax filing platform. Over the last two filing seasons, weekly new user signups grew from 420,000 to 610,000 (+45%), and first-return completion rate improved from 68% to 74% after a redesigned onboarding flow and heavier paid acquisition. Leadership is encouraged by top-line growth, but 90-day retained active rate for recent signup waves is flat at 24%, while older cohorts that grew more organically retained at 31%. You need to determine whether recent growth reflects durable user value or short-term acquisition lift ahead of planning for the next season.
How would you use cohort analysis to determine whether this growth is durable, and how would you distinguish healthy retention improvements from growth driven mainly by temporary acquisition or seasonal effects?