The conversion rate is a ratio of total conversions over total unique views. Unique views being calculated on the basis of unique users having viewed your stories and conversions being recorded after a user interacts with one of your stories and completes a purchase. Both unique views and conversions are calculated using a 7-day attribution window (a users having viewed or performed a purchase on a period of 7-day will be counted once).
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For example: if your total conversions show 10 and your total unique views 100, your conversion rate will be 10% indicating that 10% of your unique users over a period of 7 days have performed a purchase after interacting with at least one of your stories.