A system and method are described for presenting on-line survey invitations to users over a network. The system enables selectively presenting survey invitations to users in accordance with a survey recruitment definition associated with a tag embedded within an on-line advertisement downloaded and executed on a user computer. If a determination is made by a survey recruitment server that a requestor is to be a potential control (unexposed) survey participant, then instructions are issued to the requesting user computer to obstruct presentation of the on-line advertisement which caused the issuance of the request for a survey invitation from the user computer.