Official attendance might not beat the Tottenham one, which would be a shame. They may just announce it based on tickets sold though.
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According to FL rules, tickets sold,(or given away for that matter) is the official way to measure attendances. NOT people through the gate.
In England the rules of the premier league say that attendance figures that are publicly reported should be the number of tickets that are issued or sold. Not the number of people who actually attend. (I understand the football league rules say the same, although I couldn’t find the precise clause.)
The actual attendance figure can be quite different to the number of tickets that are issued or sold. Season ticket holders may not turn up for a midweek game. Complimentary, i.e. free, tickets could be issued but not used. People might even walk out in the middle of a match as a protest.
I think we can safely say that the 'attendance' is 19,841
You're generally right, the ticket (plus pay on the gate) numbers are what it is based on although I think that some unused complimentary tickets not used are discounted, this is the announced figure although you also record the number through the gates. I produced some of the figures for Forest a few years ago but I can't remember the exact method.
The figures are mainly for the use of the Football League and they are probably worried that they will get short changed.