And if you wanted to go to the next level of analysis on your historical Leads that are not members of any Marketo Programs/Events, you can create what I call "Channel Programs" -- 2010 Events, 2010 Content, 2010 Advert, etc. -- and then batch Leads into the appropriate ones with a Smart Campaign based on Create Date & Lead Source. This creates generic Acquisition Programs for Leads that pre-date your Marketo org. -- and gives Marketing visibility and ownership on even your oldest Lead records.