Your Thoughts? Does Home advantage exist?

Hi,
for a school project I have to find out some information about the home advantage in the Premier League.  So I would be extremely grateful if you could help me and answer these questions:
1.   Does the home advantage exist?
2.   Why does the home advantage exist? (Fans, pitch, etc.)
3.   (Or) is the home advantage just a psychological factor and all the external conditions have no influence?
4.   How helpful can it be to play at home? (e.g. teams from the bottom of the table can win at home against the top placed teams as a result of the home advantage?)

Thanks in advance
Chris