  Buy It Again (BIA) recommendations are crucial to retailers to help improve
user experience and site engagement by suggesting items that customers are
likely to buy again based on their own repeat purchasing patterns. Most
existing BIA studies analyze guests personalized behavior at item granularity.
A category-based model may be more appropriate in such scenarios. We propose a
recommendation system called a hierarchical PCIC model that consists of a
personalized category model (PC model) and a personalized item model within
categories (IC model). PC model generates a personalized list of categories
that customers are likely to purchase again. IC model ranks items within
categories that guests are likely to consume within a category. The
hierarchical PCIC model captures the general consumption rate of products using
survival models. Trends in consumption are captured using time series models.
Features derived from these models are used in training a category-grained
neural network. We compare PCIC to twelve existing baselines on four standard
open datasets. PCIC improves NDCG up to 16 percent while improving recall by
around 2 percent. We were able to scale and train (over 8 hours) PCIC on a
large dataset of 100M guests and 3M items where repeat categories of a guest
out number repeat items. PCIC was deployed and AB tested on the site of a major
retailer, leading to significant gains in guest engagement.
