A method of determining content to be presented on a mobile device , the method comprising: receiving sensor data from a mobile device , wherein the sensor data comprises: first sensor data relating to the positioning of the mobile device with respect to a line-of-sight between a user and a display device of the mobile device sensed by the mobile device during display of the content of a first type at the mobile device over a first time period; and second sensor data relating to the positioning of the mobile device with respect to a line-of-sight between a user and a display device of the mobile device sensed by the mobile device during display of content of a second type at the mobile device over a second time period, wherein the content of a second type is different to the content of a first type and where the first and second time periods are consecutive; and determining the comparative level of user engagement with the content of the first type based on the first sensor data and the second sensor data; selecting additional content of the first type based on the determined comparative level of user engagement; and transmitting the selected additional content of the first type to the mobile device for output by the mobile device . receiving sensor data from a mobile device , wherein the sensor data comprises: first sensor data relating to the positioning of the mobile device with respect to a line-of-sight between a user and a display device of the mobile device sensed by the mobile device during display of the content of a first type at the mobile device over a first time period; and second sensor data relating to the positioning of the mobile device with respect to a line-of-sight between a user and a display device of the mobile device sensed by the mobile device during display of content of a second type at the mobile device over a second time period, wherein the content of a second type is different to the content of a first type and where the first and second time periods are consecutive; and determining the comparative level of user engagement with the content of the first type based on the first sensor data and the second sensor data; selecting additional content of the first type based on the determined comparative level of user engagement; and transmitting the selected additional content of the first type to the mobile device for output by the mobile device . first sensor data relating to the positioning of the mobile device with respect to a line-of-sight between a user and a display device of the mobile device sensed by the mobile device during display of the content of a first type at the mobile device over a first time period; and second sensor data relating to the positioning of the mobile device with respect to a line-of-sight between a user and a display device of the mobile device sensed by the mobile device during display of content of a second type at the mobile device over a second time period, wherein the content of a second type is different to the content of a first type and where the first and second time periods are consecutive; and determining the comparative level of user engagement with the content of the first type based on the first sensor data and the second sensor data; selecting additional content of the first type based on the determined comparative level of user engagement; and transmitting the selected additional content of the first type to the mobile device for output by the mobile device . <EOS>