What does zero time-taken in IIS log file mean?

Question

What does a 0 (zero) in the time-taken field of an IIS 6 log really mean? I'm trying to get the average time-taken before and after a particular change to see if it had a positive affect, but I'm not sure how to interpret the zeros. Including/excluding them from the average gives me radically different results.

Specifically, if the application actually performed a database hit during a request, it seems virtually impossible for the time-taken to be zero. Is it reasonable to assume that a database call did not occur in that case?

Here's an example from my log file:

cs-method: GET
sc-status: 200
sc-win32-status: 0
sc-bytes: 6,966
time-taken: 0
cs(User-Agent): Mozilla/4.0+(compatible;+MSIE+8.0;+Windows+NT+5.1;+Trident/4.0;+.NET+CLR+1.1.4322;+.NET+CLR+2.0.50727;+.NET+CLR+3.0.4506.2152;+.NET+CLR+3.5.30729)

Answer 1

Are they image files, or static files? They might serve up at virtually 0 ms. Also, as Christopher G suggested, a 304 non-modified wouldn't take any time either. Also, IIS caching would allow files to be served up fast enough that it wouldn't register any time. Compression of static pages also serves as a form of caching.