1. A computer-implemented method comprising: conducting one or more experiments using a computer simulation simulating a specified future time period, the one or more experiments comprising: hypothetically serving content items to hypothetical users assumed to visit a page during the specified future time period; simulating hypothetical user behavior during the specified future time period based on observed historical user behavior data; recording, in a set of simulated data, one or more values for a performance metric for each item hypothetically served for the specified future time period; selecting a particular item of a plurality of items for presentation based, at least in part,  on the set of simulated data; and presenting the particular item to a user during an actual page visit.