[OT] tcpdump overhead

[Mario Lobo]

   Hello;
   I am doing a final paper for my post-graduate studies. The paper is
   about QOS on a corporate environment.
   One of the issues involved in the network measurements I am going to
   make is how much overhead tcpdump will have on them (if any). It seems
   to me that this overhead can be disregarded as influential but, if
   this is right, I need some "proper" source that I can quote and index
   on my paper.
   Could anyone show some directions for any "official" documentation on
   this? or maybe some opinions, sugestions from you guys on this subject
   that I could "officially" put on this paper?
   well, thanks and sorry for this off-topic.
   Mario Lobo