Email Campaign Changes

Imagine you're part of an E-commerce team. A new marketing manager has revamped the existing email journey for new users in hopes of improving conversion rates. A few weeks post-launch, the conversion rate from new user to customer climbs from 40% to 43%. However, before celebrating, the company's CMO highlights a concern. A few months before the new manager's arrival, the conversion rate was already at 45%, before gradually declining to the initial 40%. How would you determine whether the revamped email campaign was responsible for the conversion rate increase, or if other factors might have contributed to this change?