# Collaborative filtering advantages & disadvantages

## Advantages

**No domain knowledge necessary**
We don't need domain knowledge because the embeddings are automatically learned.

**Serendipity**
The model can help users discover new interests. In isolation, the ML system may not know the user is interested in a given item, but the model might still recommend it because similar users are interested in that item.

**Great starting point**

To some extent, the system needs only the feedback matrix to train a matrix
factorization model. In particular, the system doesn't need contextual features.
In practice, this can be used as one of multiple candidate generators.

## Disadvantages

**Cannot handle fresh items**

The prediction of the model for a given (user, item) pair is the dot
product of the corresponding embeddings. So, if an item is not seen
during training, the system can't create an embedding for it and can't
query the model with this item. This issue is often called the
**cold-start problem**. However, the following techniques can address
the cold-start problem to some extent:

- **Projection in WALS.** Given a new item \\(i_0\\) not seen in training,
  if the system has a few interactions with users, then the system can
  easily compute an embedding \\(v_{i_0}\\) for this item without having
  to retrain the whole model. The system simply has to solve the following
  equation or the weighted version:

  \\\[\\min_{v_{i_0} \\in \\mathbb R\^d} \\\|A_{i_0} - U v_{i_0}\\\|\\\]

  The preceding equation corresponds to one iteration in WALS: the
  user embeddings are kept fixed, and the system solves for the embedding
  of item \\(i_0\\). The same can be done for a new user.
- **Heuristics to generate embeddings of fresh items.** If the system
  does not have interactions, the system can approximate its embedding
  by averaging the embeddings of items from the same category, from the
  same uploader (in YouTube), and so on.

**Hard to include side features for query/item**

**Side features** are any features beyond the query or item ID. For movie
recommendations, the side features might include country or age. Including
available side features improves the quality of the model. Although
it may not be easy to include side features in WALS,

a generalization of WALS makes this possible.


To generalize WALS, **augment the input matrix with features** by defining a
block matrix \\(\\bar A\\), where:

<br />

- Block (0, 0) is the original feedback matrix \\(A\\).
- Block (0, 1) is a multi-hot encoding of the user features.
- Block (1, 0) is a multi-hot encoding of the item features.

<br />

> [!NOTE]
> **Note:** Block (1, 1) is typically left empty. If you apply matrix factorization to \\(\\bar A\\), then the system learns embeddings for side features, in addition to user and item embeddings.