Why “last activity” sounds simple?

Last touch before purchase is a classic question. Marketing wants attribution. Product wants behavior insights. I’ve seen teams answer it confidently — and incorrectly.

Where it usually goes wrong?

  • Events after purchase
  • Events from another session
  • Events triggered by the purchase itself

Suddenly “last interaction” happens after the conversion. What BigQuery forces you to respect?

You must clearly separate:

  • Before purchase
  • After purchase

Anything else is storytelling, not analysis.

How I approach it now

For each user, I lock the purchase timestamp. Then I only look backwards.

  • Strict time filter
  • Ordered events
  • One last valid action

That’s when last-touch analysis starts making sense.

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