Hi Lee,
I understand now. Not as easy as I first thought and getting beyond my capabilities Excel wise to do a random selection for thirteen numbers over a 20 week period without (or least as little as possible) duplication.
There is a way but it involves the MRAND function which requires "Add ins" and downloading from another site and I would never tell anybody to do this although I'm quite comfortable with it. There is a demo on YouTube that you can look at and it is almost what you need. http://www.youtube.com/watch?v=rYrGF8ib7WY
However, I have been giving your problem some thought and I've come up with a matrix which you can complete each week and possibly goes a bit further by way of rewarding the kids rather than randomising the awards.
The attached allows for you to enter the players number and on the right it will keep a tally of who has had what. It enables you to make a considered choice without fear of missing anybody out.
From an Excel point of view I think it's beyond me although it surely can be done and if you feel like reposting this perhaps you could rename it something like "Randomised selection without duplication".
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