When your solutions finish creating, the next step is to obtain the evaluation metrics. Personalize calculates these metrics based on a subset of the training data. The image below illustrates how Personalize splits the data. Given 10 users, with 10 interactions each (a circle represents an interaction), the interactions are ordered from oldest to newest based on the timestamp. Personalize uses all of the interaction data from 90% of the users (blue circles) to train the solution version, and the remaining 10% for evaluation. For each of the users in the remaining 10%, 90% of their interaction data (green circles) is used as input for the call to the trained model. The remaining 10% of their data (orange circle) is compared to the output produced by the model and used to calculate the evaluation metrics.

We recommend reading the documentation to understand the metrics, but we have also copied parts of the documentation below for convenience.

You need to understand the following terms regarding evaluation in Personalize:

*Relevant recommendation*refers to a recommendation that matches a value in the testing data for the particular user.*Rank*refers to the position of a recommended item in the list of recommendations. Position 1 (the top of the list) is presumed to be the most relevant to the user.*Query*refers to the internal equivalent of a GetRecommendations call.

The metrics produced by Personalize are:

**coverage**: The proportion of unique recommended items from all queries out of the total number of unique items in the training data (includes both the Items and Interactions datasets).**mean_reciprocal_rank_at_25**: The mean of the reciprocal ranks of the first relevant recommendation out of the top 25 recommendations over all queries. This metric is appropriate if you’re interested in the single highest ranked recommendation.**normalized_discounted_cumulative_gain_at_K**: Discounted gain assumes that recommendations lower on a list of recommendations are less relevant than higher recommendations. Therefore, each recommendation is discounted (given a lower weight) by a factor dependent on its position. To produce the cumulative discounted gain (DCG) at K, each relevant discounted recommendation in the top K recommendations is summed together. The normalized discounted cumulative gain (NDCG) is the DCG divided by the ideal DCG such that NDCG is between 0 - 1. (The ideal DCG is where the top K recommendations are sorted by relevance.) Amazon Personalize uses a weighting factor of 1/log(1 + position), where the top of the list is position 1. This metric rewards relevant items that appear near the top of the list, because the top of a list usually draws more attention.**precision_at_K**: The number of relevant recommendations out of the top K recommendations divided by K. This metric rewards precise recommendation of the relevant items.

Let’s take a look at the evaluation metrics for each of the solutions produced in your notebook.

For additional reading you should check out this A/B testing blog!