A method, comprising: at an electronic device with a display and a touch-sensitive surface, wherein the device includes a plurality of intensity sensors (12902-1 - 12902-4) to detect intensities of contacts with the touch-sensitive surface: detecting, on the touch-sensitive surface, a plurality of contacts; while detecting the plurality of contacts: receiving a first intensity measurement from a first intensity sensor of the plurality of intensity sensors; receiving a second intensity measurement from a second intensity sensor of the plurality of intensity sensors, wherein the first intensity sensor is different from the second intensity sensor; assigning a first portion of an aggregate intensity of the contacts to a first contact and assigning a second portion of the aggregate intensity to a second contact based at least in part on: the first intensity measurement; the second intensity measurement; a location of the first intensity sensor relative to the touch-sensitive surface; a location of the second intensity sensor relative to the touch-sensitive surface; a comparison between values of a set of one or more properties of the first contact and corresponding values of the set of one or more properties of the second contact; and, in accordance with a determination that the plurality of contacts includes an incidental contact that is distinct from the first contact and the second contact, cancelling an estimated contribution of the incidental contact to the first intensity measurement and the second intensity measurement; and in response to detecting the plurality of contacts, performing an operation based at least in part on the portion of the aggregate intensity assigned to the first contact. detecting, on the touch-sensitive surface, a plurality of contacts; while detecting the plurality of contacts: receiving a first intensity measurement from a first intensity sensor of the plurality of intensity sensors; receiving a second intensity measurement from a second intensity sensor of the plurality of intensity sensors, wherein the first intensity sensor is different from the second intensity sensor; assigning a first portion of an aggregate intensity of the contacts to a first contact and assigning a second portion of the aggregate intensity to a second contact based at least in part on: the first intensity measurement; the second intensity measurement; a location of the first intensity sensor relative to the touch-sensitive surface; a location of the second intensity sensor relative to the touch-sensitive surface; a comparison between values of a set of one or more properties of the first contact and corresponding values of the set of one or more properties of the second contact; and, in accordance with a determination that the plurality of contacts includes an incidental contact that is distinct from the first contact and the second contact, cancelling an estimated contribution of the incidental contact to the first intensity measurement and the second intensity measurement; and receiving a first intensity measurement from a first intensity sensor of the plurality of intensity sensors; receiving a second intensity measurement from a second intensity sensor of the plurality of intensity sensors, wherein the first intensity sensor is different from the second intensity sensor; assigning a first portion of an aggregate intensity of the contacts to a first contact and assigning a second portion of the aggregate intensity to a second contact based at least in part on: the first intensity measurement; the second intensity measurement; a location of the first intensity sensor relative to the touch-sensitive surface; a location of the second intensity sensor relative to the touch-sensitive surface; a comparison between values of a set of one or more properties of the first contact and corresponding values of the set of one or more properties of the second contact; and, in accordance with a determination that the plurality of contacts includes an incidental contact that is distinct from the first contact and the second contact, cancelling an estimated contribution of the incidental contact to the first intensity measurement and the second intensity measurement; and the first intensity measurement; the second intensity measurement; a location of the first intensity sensor relative to the touch-sensitive surface; a location of the second intensity sensor relative to the touch-sensitive surface; a comparison between values of a set of one or more properties of the first contact and corresponding values of the set of one or more properties of the second contact; and, in accordance with a determination that the plurality of contacts includes an incidental contact that is distinct from the first contact and the second contact, cancelling an estimated contribution of the incidental contact to the first intensity measurement and the second intensity measurement; and in response to detecting the plurality of contacts, performing an operation based at least in part on the portion of the aggregate intensity assigned to the first contact.