Methods and apparatus to monitor online activity

A non-transitory computer-readable storage medium, having stored thereon program instructions that, upon execution by a processor, cause performance of a set of operations comprising:

• obtaining first uniform resource locators collected from first client device that are associated with panelists;
• obtaining second uniform resource locators collected from second client devices, wherein the second client devices are associated with one or more unidentified users;
• associating at least one of the second uniform resource locators as online activity of a panelist of the panelists based on at least a portion of one of the first uniform resource locators matching at least a portion of the one of the second uniform resource locators;
• associating the online activity with an identifier of the panelist;
• and
• storing information corresponding to an association of the online activity and the identifier.

A method comprising:

• obtaining first uniform resource locators collected from first client devices that are associated with panelists;
• obtaining second uniform resource locators collected from second client devices, wherein the second client devices are associated with one or more unidentified users;
• associating at least one of the second uniform resource locators as online activity of a panelist of the panelists based on at least a portion of one of the first uniform resource locators matching at least a portion of the one of the second uniform resource locators;
• and
• updating a panelist record of the panelist based on an association of the online activity and an identifier of the panelist.