Average days between purchases (ADP), aka. interpurchase time (IPT), tells us on average how many days pass between orders for a given segment of customers. For particular segments, ADP is useful for optimizing marketing channel timing, and experimenting with moving this number (usually lower).
In Segments, we use non-fully-refunded orders to calculate ADP.
ADP = Sum(Days between orders) / Orders
In Shopify, ADP can be derived from subtotal_price and created_at.