In the acquisition of a company, the buyer sometimes or oftentimes
holds back a percentage of the purchase price for a short to medium
period of time just in case the seller made misrepresentations, etc. 
I need to determine two things: 1) whether it is common or uncommon to
have a holdback, and 2) what is the typical holdback range as a
percentage of the purchase price.  If the answer is dependent on the
size of the acquisition, I am looking at a deal under $50 million. 
The answer should be referenced with links from a reasonably reliable
third party.  Thanks!!!