As online groups grow in number and type, understanding lurking is becoming increasingly important. Recent reports indicate that lurkers make up over 90% of online groups, yet little is known about them.  This paper presents a demographic study of lurking in email-based discussion lists (DLs) with an emphasis on health and software-support DLs. Four primary questions are examined. One, how prevalent is lurking, and do health and software-support DLs differ? Two, how do lurking levels vary as the definition is broadened from zero posts in 12 weeks to 3 or fewer posts in 12 weeks? Three, is there a relationship between lurking and the size of the DL, and four, is there a relationship between lurking and traffic level?  When lurking is defined as no posts, the mean lurking level for all DLs is lower than the reported 90%. Health-support DLs have on average significantly fewer lurkers (46%) than software-support DLs (82%). Lurking varies widely ranging from 0 to 99%. The relationships between lurking, group size and traffic are also examined.
['Blair Nonnecke', 'Jennifer Preece']
Lurker demographics: counting the silent