FUNNEL v1
Question:where does recorded new-user progression stop?
Unit:user_id. Population:12 non-internal signup users with an observed earliest start.
Events:strictly ordered start<lesson<exercise, final time<=first start+72h.
One first-start attempt per user; later starts do not reset. No session restriction.
All entrants must have complete72h observation; reference refuses incomplete windows.
Numerator:5 reaching exercise. Denominator:12 reaching start. Steps12,8,5.
Overall41.67%; conditional66.67%,62.50%. No claims about motivation/causality.

RETENTION v1
Unit:user_id; cohorts fixed Monday UTC signup week;4 users each, internal excluded.
Return event:practice_active. Dayd means elapsed[signup+d days,signup+(d+1)days).
At most one return per user/interval. Original cohort denominator retained.
Publish cohort row only when all intervals complete; otherwise count/rate=null/N/A.
Watermark2026-01-26T00:00Z exclusive, source completeness assumed by fictional brief.
Day 1:3/4,2/4,1/4. Day 7:2/4,1/4,N/A. Pool mature counts3/8, not 3/12.
Day 8 returns do not count for exact day 7. No rolling-retention claim.
