Growth Rate Is the Measure, Weekly Is the Unit

The best thing to measure is revenue, and if you're not charging yet, active users. What you track is the growth rate: the ratio of this week to last week, not the absolute number of new users.

Absolute numbers deceive. A constant 100 new customers a week sounds steady, but as your base grows it's a shrinking percentage: your growth rate is decaying. Ratios tell the truth.

Why weekly? Because early-stage startups need frequent feedback, and because a week is short enough to connect cause and effect. YC measures companies by weekly growth for exactly this reason.