Co-occurrence

Co-occurrence measures how frequently two products appear together in the same context — e.g., the same basket, order, or browsing session. The more often two products co-exist, the stronger their association score.

How it is calculated

A co-occurrence matrix C is built where each cell C[i,j] represents the number of times product i and product j appear together.

	iPhone	Case	Charger	Book
iPhone	—	250	180	2
Case	250	—	40	1
Charger	180	40	—	0

The association iPhone → Case is much stronger than iPhone → Book.

This raw count can be normalized, for example using a cosine-style similarity score:

Example

• A customer frequently buys an iPhone together with a case and a charger → “Frequently bought together” block.

• A shopper browses backpacks and laptops in the same session → “Co-viewed products” logic connects them.

Key takeaways

• Fully based on real user behavior (purchase or browsing).

• Simple, interpretable, and very effective once there’s enough volume.

• Less reliable for new products with no data yet.