Here in Germany, official average is based upon last season's recorded league or tournament results. But you have to have played at least 24 games to receive an offcial average which is then noted on your player pass for the upcoming season.
Additionally, depending on the governing body you belong to, you might have another member card with a different average - that's in my case the result of my house league.
Furthermore, I have an "officail" training average at my club which take sinto account any officail and submitted training game of the running year (not season). The training limit is that to submit the games at least 3 club members must have played together, just to make sure that the numbers are "true". Most of these games come from my club's lane, and from traveling league.
And, lastly, I keep my personal record of any game I made this year, and keep up the overall average. For me, this one is the most relevant, together with the "official" tournament average from my player card. The training average from my club tends to be too high, I think, because I know the lane very well. Any my house league average is not representative, because the lanes are awful - even though it might be taken for "desert condition" tournament average as reference.
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DizzyFugu - Reporting from Germany
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