Techniques are provided for building a unified model for selecting content items of different types in response to receiving electronic content requests transmitted over a network. In one technique, in response to a request, multiple content items are identified. The multiple content items include a first content item of a first type and a second content item of a second type. A first engagement value that indicates a first level of engagement of an online resource for content items of the first type is determined. A first predictive user selection rate is generated for the first content item based on the first engagement value. A second predictive user selection rate is generated for the second content item. The multiple content items are ranked based, at least in part, on the predictive user selection rates. A particular content item is then selected based on the predictive user selection rates.
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