Ensemble learning boosting 

spam email {+, -}
sample rules: body “manly” +, from spouse -, short +, just URLs +, just image +, “pΘrn” +, “make money easy” +

1. Learn over a subset of data -> rule: uniformly randomly, pick data and apply a learner
2. combine: complex rule -> mean

Boosting
“hardest” examples
weighted mean

Error: mismatches
PrD[h(x) = c(x)]