Abstract: An online system provides content based on context of host content. The online system allows content providers to define specific context for providing their content for display to users, including specifying categories, format, popularity, and visual appearance of host content eligible for hosting their content. To determine which content a specific content item is eligible for hosting, the online system categories host content items according to specific criteria defined by different content providers for hosting various content. Each content is indexed by classification groups into which the host content items are classified. If a particular content item includes an opportunity for presenting content, the online system identifies content which the particular content item is eligible for hosting. The online system selects one or more identified content for presentation via the opportunity.

Abstract: In one embodiment, a method includes a computer server machine receiving a request from a first user to interact with a multimedia content. The computer server machine then associates the first user with a control group, wherein the control group comprises a first set of users interacting with the multimedia content. When the first user is associated with the control group, the computer server machine then applies a first content insertion model to the multimedia content and records a first set of metrics based on the interaction of the first user with the multimedia content.