1. A method comprising: specifying parameters that describe how users behave in an absence of advertising, parameters that describe how users react to advertising, and parameters that regulate injection of advertising in a simulation, wherein specifying parameters that describe how users behave in an absence of advertising includes at least one of specifying an activity level that influences a length of a simulated activity stream, an activity preference from among a plurality of possible activities, and a conversion rate for a given activity, wherein specifying parameters that describe how users react to advertising includes specifying a probability that a user's behavior will change with advertising exposure, wherein the simulation is conducted using the specified parameters to generate a plurality of simulated activity streams, each simulated activity stream representing a sequence of simulated online actions performed by a virtual user during a simulated web session; generating, by one or more processors, a first set of simulated activity streams with advertising turned on, based on the specified parameters; determining observational metrics associated with the first set of simulated activity streams; applying an attribution model to the generated first set of simulated activity streams and determining a first fraction of incremental conversions associated with one or more advertising channels; generating, by one or more processors, one or more further sets of simulated activity streams, each with a single advertising channel turned off, based on the specified parameters; determining observational metrics associated with the one or more further sets of simulated activity streams; determining a second fraction of incremental conversions for each advertising channel by evaluating the further set of simulated activity streams with the single advertising channel turned off; and comparing the first and second fractions of incremental conversions to determine an applicability of the attribution model for estimating effectiveness of a campaign, wherein the comparing comprises generating a comparison report that includes information related to the first and second fractions and providing the comparison report for presentation at a client device.