An identifier associated with a client machine is accessed, and parameters that define a control group of client machines for a particular advertising campaign are accessed. The advertising campaign is associated with one or more standard advertisements. Whether the client machine is a member of the control group based on the identifier associated with the client machine and the parameters that define the control group is determined. If the determination of whether the client machine is a member of the control group indicates that the client machine is a member of the control group, a control advertisement is caused to be presented on the client machine. The control advertisement is an advertisement not associated with the advertising campaign.