1. A system configured to estimate a response to a token instance of interest, comprising: a memory configured to store: token instances to which a user was exposed, a total response of the user to the token instances, and at least two respective representations of attention level of the user in at least two of the token instances; wherein the at least two of the token instances have overlapping instantiation periods; a processor configured to utilize the representations of attention level for selecting the token instance of interest from among the at least two of the token instances; and the processor is further configured to estimate the response to the token instance of interest from the total response.