I am looking for statistics on international tourism.  I am trying to
understand how many people in a given year, are travelling as a
'backpacker'.  An acceptable definition of a backpacker would be
'someone who is away from home for 2+ months travelling', 'someone who
spends one or more nights in a Youth Hostel' or someone who travels as
a 'Fully
Independent Traveller'.  Where possible I want to avoid double
counting - ie if someone goes backpacking in 2 countries, I do not
want
them counted twice in the overall statistics

You could try asking this question at http://www.green-travel.com/ .