# Time to Next Event

The final topic in this chapter combines the ideas from survival analysis with repeated events. This topic is quite deep, and this section is just an introduction. The question is: *How long until a customer places another order?*

## Idea behind the Calculation

To apply survival analysis to repeated events, each order needs the date of the next order in the household (if any). The order date and next order date provide the basic information needed for time-to-event survival analysis. The definitions for survival analysis are inverted from the last two chapters:

- The “start” event is when a customer makes a purchase.
- The “end” event is either the next purchase date or the cutoff date.

This terminology for repeated events is backward. “Survival” ends up meaning the survival of the customer’s “non-purchase” state. In fact, we are interested in the exact opposite of survival, 100%-Survival, which is the cumulative...