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**JakeD** The 95% margin of error m for a proportion p is m = 2*sqrt(p(1-p)/n) when n is the number of events. For the sides bet, m = 2*sqrt(.41(1-.41)/204) = .07. What you can say is that 95% of the time, the true proportion will be in the interval generated by the formula [p-m,p+m]. For p = .41 and n = 204, that interval is [.34,.48]. How confident does that make you?

I think the biggest problem with betting systems is not about the data, it is that the behavior of bettors, NBA players, coaches, etc, changes in response to what happened in the past. So a system that worked successfully against them in the past, very likely will not continue to work against them in the future.