Replacement

In some cases, read items should be recommended to users again but with lower (or higher) priorities.

Configuration

Replacement is configured in the [recommend.replacement] section of configuration file:

• enable_replacement enables or disables replacement of read items back to recommendations. Defaults to false.
• positive_replacement_decay is the decay factor for ranking replaced items from positive feedback. Defaults to 0.8.
• read_replacement_decay is the decay factor for ranking replaced items from read feedback. Defaults to 0.6.

Set decay factor greater than 1.0 if you want to increase the priority of read items.

Algorithm

If enable_replacement is set to true:

1. Gorse collects read items from user feedback.
2. Gorse adds read items back to the recommendation list with decayed scores. For an item $i$ read by user $u$, if the original score from the ranker is $\hat y_{ui}$, the new score after replacement is

$$\hat y'_{ui}=\begin{cases} \hat y_{ui}\cdot \lambda_p & i\in R^p_u\\ \hat y_{ui}\cdot \lambda_r & i\in R^r_u\\ \hat y_{ui}&else \end{cases}$$

where $R^p_u$ is the set of items with positive feedback from user $u$, $R^r_u$ is the set of items with read feedback from user $u$, $\lambda_p$ is the positive_replacement_decay and $\lambda_r$ is the read_replacement_decay.