Another major concern with any spontaneous reporting system is underreporting of adverse events (16, 30-32). It has been estimated that rarely more than 10% of serious ADRs, and 2-4% of non-serious reactions, are reported to the British spontaneous reporting program (30). A similar estimate is that the FDA receives by direct report less than 1% of suspected serious ADRs(32). This means that cases spontaneously reported to any surveillance program, which comprise the numerator, generally represent only a small portion of the number that have actually occurred. The effect of underreporting can be somewhat lessened if submitted reports, irrespective of number, are of high quality.
http://www.fda.gov/medwatch/articles/medcont/postrep.htm LINK NO LONGER VALID
I had to redo the search for the information and was able to locate it here,
You may have to copy/paste link
(updated October 19.2016)