Many leagues in my home center uses averages in the following order:
1)Winter Book from the same league with a min number of game, 60 I believe
2)House book with a minimum number of games, generally 40 games
3)High Winter Book (21 games minimum)
4)Summer book (even though there is not an official book put together for the summer, averages are compiled and can be found by contacting your locl association or checking bowl.com)
5) 2 Year Winter Book
Then there is always the "If none of the above use " and based on the league a number is inserted, usually the vacant score used by the league.
The league and house book are used whether you were a regular paying member or a sub as long as you meet the games requirement.
My sport league gets really detailed and there is a long line of succession to try to make it fair for new members who do not have a sport average.
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USBC Bronze Coach "I cannot change the direction of the wind but I can adjust my sails to reach my destination." Jimmy Dean
Quaker 10/93 - 4/07
Quaker