Why cumulative metrics are tempting?

Cumulative counts inside a session look powerful. They help understand pacing, friction, and behavior. I’ve used them a lot — and broken them a few times.

Where things quietly go wrong?

If you don’t lock session boundaries, cumulative metrics leak.

  • One session spills into another
  • Counts keep growing
  • Insights look dramatic — and false

Cumulative logic requires discipline

  • Strict session partition
  • Correct event order
  • No shortcuts

Window functions amplify mistakes if boundaries are wrong.

How I keep it safe?

I always partition by session and order by time. No exceptions. That’s the difference between insight and fiction.

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