def part_key(exp, alt, date_hour=nil)
      parts = [ "participants", exp.name, alt ]
      parts += [ date_hour.date, date_hour.hour ] unless date_hour.nil?
      make_key parts
    end
def part_key(exp, alt, date_hour=nil)
      parts = [ "participants", exp.name, alt ]
      parts += [ date_hour.date, date_hour.hour ] unless date_hour.nil?
      make_key parts
    end
if date_hour is nil, create key for total
 otherwise, create key for hourly based